April 19 and 20 seem to be the unluckiest dates ever. Waco, Oklahoma City, and Columbine will sear it forever in our hearts and minds. Add to that the assassination of Lincoln and the Titanic sinking and there is no other date on the calendar that even comes close. Here is my list of tragedies and important events that occurred on those dates or surrounding dates. (Not all of these dates are notable and this needs editing).
April 23, 303 AD - Death of St. George. St. George is the patron saint of England, and St. George's Day is celebrated as a major holiday there.
April 17, 1421 - Dikes at Dort Holland breaks, 100,000 drown
April 17, 1457 - Vlad the Impaler (aka Dracula) executes boyers at an Easter feast in Wallachia
April 17, 1521 - Martin Luther refuses to recant his teachings at the Diet of Worms
April 23, 1616 - Death of William Shakespeare. He may have also been born on April 23 in 1564
April 20, 1653 - Oliver Cromwell dissolves the Rump Parliament
April 21 (o.s. May 2 n.s.), 1729 - Catherine the Great of Russia born
April 19, 1770 - Capt. James Cook discovers Australia
April 19, 1775 - American Revolution begins in Lexington, Massachusetts with the "shot heard round the world"
April 19, 1782 - The Dutch Republic became the first foreign nation to recognize U.S. independence when it accepted John Adams as ambassador. April 19 is still celebrated as Dutch-American Friendship Day
April 20, 1792 - France declares war on Austria, beginning the French Revolutionary Wars, which continue until 1802
April 19, 1810 - Venezuela becomes independent of Spain
April 21, 1836 - The Battle of San Jacinto won by the Texas army led by Gen. Sam Houston. It was the deciding battle in the Texas Revolution.
April 19, 1839 - Belgium independence is recognized by the Treaty of London
April 18, 1857 - Clarence Darrow, a lawyer who defended John Scopes in 1925 for teaching evolution, is born
April 12, 1861 - Civil War begins as Confederates fire on Ft. Sumter
April 19, 1861 - riot in Baltimore between Confederate sympathizers and the Massachusetts militia
April 20, 1861 - Robert E. Lee resigns from the US Army in order to command the forces of Virginia
April 14, 1865 - Abraham Lincoln shot by John Wilkes Boothe and dies on April 15
April 22 (April 10 o.s.), 1870 - Birthday of Vladimir Lenin
April 20, 1876 - The April Uprising occurs in Bulgaria, which resulted in the establishment of Bulgaria as an autonomous nation
April 22 (April 10 o.s.), 1899 - Birthday of Vladimir Nabokov, author of the novel Lolita
April 18, 1881 - Billy the Kid escapes from jail in Mesilla County, NM
April 19, 1882 - death of Charles Darwin
April 30, 1888 - 230 killed by hail the size of oranges in Moradabad, India
April 22, 1889 - Oklahoma land rush begins
April 20, 1889 - Birth of Adolf Hitler
April 19, 1904 - The Great Fire of Toronto
April 18, 1906 - earthquake destroys much of San Francisco
April 14, 1907 - Haitian dictator Francois "Papa Doc" Duvalier born in Port-au-Prince
April 23, 1907 - death of Alferd Packer the cannibal
April 21, 1910 - Mark Twain dies
April (unknown year). Fictional sinking of the ocean liner Titan which struck an iceberg in the novel “The Wreck of the Titan, or Futility” published in 1898, which was very similar to the real-life sinking of the Titanic in 1912
April 15, 1912 - Titanic strikes an iceberg and sinks
April 20, 1912 - Bram Stoker, author of Dracula, dies
April 20, 1914 - Ludlow massacre, in which 20 people were killed by the Colorado National Guard
April 24, 1915 - Armenian Genocide begins with the total number of deaths between 1 and 1.5 million
April 25, 1915 - 100,000 soldiers die in a failed attack on Gallipoli, Turkey during WWI. It is remembered as ANZAC day.
April 21, 1918 - Manfred von Richthofen aka the "Red Baron" was killed in action
April 20, 1920 - Tornadoes kill 219 in Alabama and Mississippi
April 25, 1920 - San Remo conference incorporates Balfour Declaration and makes Palestine a British Mandate
April 21, 1926 - Birth of Elizabeth Windsor II, Queen of England
April 19, 1936 - Arab revolt begins in Jaffa, Palestine against Jewish immigrants
April 21, 1936 - Birth of James Dobson, American psychologist
April 20, 1937 - George Takei, who plays Mr. Sulu in Star Trek, born
April 1940 In Katyn forest in Russia, 22,000 Polish officials were executed by the Soviet secret police
April 19, 1940 - "Lake Shore Ltd" derails speed killing 34 near Little Falls NY
April 23, 1940 - Rhythm nightclub fire in Natchez, Mississippi kills 209 African-Americans
April 18, 1942 - Doolittle Raid on Japan
April 19, 1943 - beginning of Warsaw Ghetto Uprising. " Nazi Storm Troopers began to pour fire into each end of the storm drain, using flame-throwers"
April 16, 1945 - More than 6,000 die when the German refugee ship Goya is sunk by a Soviet submarine torpedo
April 29, 1945 - Adolf Hitler marries Eva Braun
April 30, 1945 - Adolf Hitler commits suicide
April 17, 1946 - Syria becomes independent of France
April 20, 1946 - The League of Nations dissolves, giving its remaining property and power to the United Nations
April 16, 1947 - Texas City Disaster: An explosion on board a freighter in port causes the city of Texas City, Texas, to catch fire, killing almost 600.
April 23, 1947 - Phil Schneider born. He lectured on alien abductions, underground bases, the OKC bombing, etc, and died in mysterious circumstances in January 1996
April 18, 1949 - Republic of Ireland comes into force
April 16, 1951 - British submarine Affray sank in English Channel, killing 75
April 19, 1960 - Revolution in South Korea
April 19, 1961 - Bay of Pigs invasion of Cuba fails
April 12, 1961 - Yuri Gagarin becomes first human to travel into space and orbit the Earth
April 4, 1968 - Martin Luther King, Jr. assassinated
April 20, 1968 - Enoch Powell of the UK, gives an incindiary racist "Rivers of Blood" speech, criticizing immigration and anti-discrimination policies
April 20, 1968 - South Africa Boeing 707 crashes at Windhoek, 122 killed
April 23, 1968 - birth of Oklahoma city bomber Timothy McVeigh
April 17, 1969 - Sirhan Sirhan was convicted of the murder of Robert Kennedy.
April 23, 1969 - Sirhan Sirhan was sentenced to death by gas chamber, later commuted to life in prison.
April 17, 1970 - The ill-fated Apollo 13 spacecraft returns to Earth
April 21, 1970 - Principality of Hutt River declares its independence from Australia
April 22, 1970 - environmental activists celebrate Earth Day
April 19, 1971 - Veterans begin a protest against the Vietnam war in Washington DC.
April 19, 1971 - Charles Manson was sentenced to death. This was later changed to life in prison.
April 21, 1971 - Haitian dictator Francois "Papa Doc" Duvalier dies
April 22, 1971 - John Kerry testifies for 2 hours against the Vietnam War before the Senate.
April 20, 1972 - Apollo 16 lands on the moon
April 21, 1975 - Battle of Xuan Loc. In the last major battle of the Vietnam war, the South Vietnamese army is defeated, allowing the North to advance on Saigon.
April 23, 1977 - Miltitary workers kill 300-500 students in Addis Ababa
April 20, 1978 - Korean Airlines Flight 902 shot down by Soviet aircraft
April 18, 1980 - Zimbabwe comes into being. It was formerly known as Rhodesia
April 20, 1980 - hundreds of Berber activists arrested in Algeria, in a critical point during the Berber Spring
April 23, 1980 - Soviet sub catches fire off Japan, 9 die
March 30, 1981 - attempted assassination of Ronald Reagan by John Hinckley, Jr.
April 19, 1981 - Hayden Christensen, actor who plays Anakin Skywalker in Star Wars, is born
April 18, 1983 - U.S. embassy in Lebanon bombed by Islamic terrorists, killing over 60 people
April 20, 1984 - during the Good Friday Massacre, the Montreal Canadiens and Quebec Nordiques get into a brawl that lasted for a full hour
April 19, 1985 - ATF siege of Aryan compound in Arkansas, ending peacefully
April 5, 1986 - Libyan agents bomb a Berlin disco, killing 4 and injuring 230
April 15, 1986 - US military bombs Libya in response to the Berlin bombing
April 26, 1986 - Chernobyl disaster, worst nuclear disaster in history
April 21, 1987 - terrorist attack by Tamil Tigers in Colombo, Sri Lanka, killing 106
April 23, 1987 - 28 construction workers killed in an apt collapse in Bridgeport, Ct
April 9, 1989 - Soviet troops fire on protestors in Tbilisi, Georgia, killing 20, leading to Georgia's independence. It is celebrated today as a public holiday.
April 15, 1989 - Hillsborough, UK disaster, killing 96 people in a stampede at a football match. It is the deadliest stadium-related disaster in British history.
April 15, 1989 - Hu Yaobang, Chinese reformer, dies of a heart attack, and 100,000 gather to mourn, leading to the Tiananmen Square protests
April 19, 1989 - gun turret on the USS Iowa explodes, killing 47 sailors, in an act of sabotage
April 5, 1991 - Sen. John Tower was killed in an suspicious airplane crash near Brunswick, Georgia
April 10, 1992 - IRA bombs the Baltic Exchange building in London, killing 3 and causing £800 million in damages
April 22, 1992 - Guadalajara, Mexico explosion in sewer system kills over 200
April 29, 1992 - Rodney King riots in Los Angeles, killing 53 people and causing over $1 billion in damages
April 19, 1993 - Waco tragedy - 76 people, including cult leader David Koresh, died in a fire after a siege by the ATF
April 19, 1993 - George Mickelson, governor of S. Dakota, dies in a plane crash near Dubuque, Iowa along with 7 others. This was the same date as the end of the Branch Davidian siege.
April 19, 1993 - Fire in psychiatric institute in South Korea, kills 40
April 24, 1993 - Bishopsgate bombing in London, killing 1 and causing over £1 billion in damages, near the Baltic Exchange bombing the previous year
April 6, 1994 - Rwandan Genocide begins, killing at least 500,000
April 20, 1994 - Serbian army bombs hospital in Goradze Bosnia, 47 killed
April 22, 1994. Former Pres. Richard Nixon dies at the age of 81
April 19, 1995 - Oklahoma City bombing by Timothy McVeigh killing 168 people
April 19, 1995 - white supremacist Richard Snell executed in Arkansas
April 3, 1996 - US Sec. of Commerce Ron Brown is killed in a suspicious plane crash
April 18, 1996 - In Qana, Lebanon, Israeli forces shell a UN compound, killing at least 106 civilians
April 27, 1996 - Former CIA director William Colby dies in a suspicious boating accident
April 18, 1997 - Red River Flood begins with a dike breached in Grand Forks, ND
April 24, 1997 - trial of Timothy McVeigh begins in Denver
April 19, 1999 - The German Bundestag returns to Berlin
April 20, 1999 - Columbine High School massacre, killing 15 including the assailants
April 18, 2000 - Zimbabwe President Robert Mugabe vilifies white farmers as enemies
April 19, 2000 - Air Philippines flight 541 crashes into a hill near Davao, killing 131
April 22, 2000 - six-year old Elian Gonzales seized by Federal agents and sent to Cuba
April 23, 2000 - Gulf Air flight 72 crashes near Bahrain, killing 143
April 28, 2000 - Carlos Ghigliotti, who was investigating the Waco incident, died of an apparent heart attack at his home in Washington DC
April 18, 2002 - a plane crashes into a tower in Milan, Italy killing 3 and injuring 60
April 22, 2004 - Train fire and explosion in Ryongchon, North Korea kills at least 160 and levels several buildings
April 22, 2004 - Pat Tillman, NFL star, was killed or possibly assassinated in a "friendly fire" incident in Afghanistan
April 6, 2005 - White Supremacist Matthew Hale sentenced to 40 years in prison for trying to kill a federal judge
April 19, 2005 - Cardinal Ratzinger, former Nazi, elected pope and becomes Pope Benedict XVI
April 16, 2007 - Virginia Tech massacre, killing 33 including the assailant Seung-Hui Cho
April 20, 2007 - Johnson Space Center shooting in which the gunman killed one person before committing suicide
April 23, 2007 - death of former Russian President Boris Yeltsin
April 20, 2008 - more than 10,000 people gather at CU-Boulder to smoke marijuana and celebrate "420", a counterculture holiday
April 29, 2008 - Roland Carnaby, possible CIA agent, was killed by Houston police after a high speed chase
April 12, 2009 - The Zimbabwean Dollar is abandoned, as its value had disappeared because of massive hyperinflation
April 19, 2009 – Easter (varies from year to year)
April 21, 2009 - Holocaust Remembrance Day (varies from year to year)
April 10, 2010 - The Polish president and 95 other people died in an airplane crash in Russia en-route to commemorate the 70th anniversary of the Katyn massacre
April 14, 2010- Neo-Nazi Bill White sentenced to 30 months in prison
April 14, 2010 - Eruption of a volcano in Iceland which severely disrupted airline traffic in Europe for a week.
April 18, 2010 - US Pres. Obama was forced to cancel a trip to Poland to attend the funeral of the Polish president due to the volcanic plume covering Europe
April 19, 2010 - In Canada over 400 hundred of straw bales caught on fire and blazed out of control.
April 20, 2010 - Deepwater Horizon oil rig explodes in the Gulf of Mexico, killing 11 workers and causing a massive spill covering at least 2500 square miles
April 11, 2011 – The accident at the Fukushima Nuclear Power Station is officially raised to Level 7, which is the same level as the Chernobyl disaster.
April 15, 2011 – Income tax due
April 15, 2011 – Atlas Shrugged, the movie based on the book by Ayn Rand, opens
April 18, 2011 - A landslide in Indonesia's East Java province kills 100 people
April 19, 2011 - Fidel Castro resigns as dictator of Cuba
April 20, 2011 - A pipe bomb is found at Southwest Plaza Mall, only 1 mile from Columbine
April 20, 2011 - Neo-Nazi Bill White has his conviction for promoting violence on his website overturned as a violation of his First Amendment rights
April 21, 2011 - Students at Stony Creek Elementary School, about 2 miles from Columbine, were evacuated due to reports of a suspicious man in the building. The strange man was there because he had to use the restroom.
Update: I just clicked on the article about The Covenant, The Sword, and the Arm of the Lord, which states: "The date of the siege [April 19, 1985], coincidentally, was the 210th anniversary of "the shot heard round the world" from the Revolutionary War. Eight years later in 1993, this was the date that the FBI chose to end the standoff in Waco, after specifically having studied the outcome of the CSA standoff. Two years after that, in 1995, this was the date chosen by Timothy McVeigh, in protest against the Waco incident among others, to bomb the Federal building in Oklahoma City."
The Columbine High School Massacre page states: "One theory states that the original date chosen was April 19, because it was a date on which Robyn Anderson, one of the people who purchased the guns and a close friend of Klebold, would not be present. Due to delays in manufacturing the propane bombs and in acquiring ammunition, the date was moved to April 20. The date of the shooting coincided with Adolf Hitler's birthday, leading to some speculation that the two were Neo-Nazis. It was also one day after the anniversary of the Oklahoma City Bombing and the immolation of the Branch Davidians in Waco, further strengthening another theory that the original intended date was set for April 19; both shooters mentioned in videos that they had hoped to outdo these events."