September surplus

The US government usually runs a surplus in September.
In September 2018, the surplus was $116 billion ($343 B receipts vs $227 B outlays).
In September 2017, the surplus was $8 billion ($349 B receipts vs $341 B outlays).
In September 2016, the surplus was $33 billion ($358 B receipts vs $325 B outlays).
In September 2015, the surplus was $91 billion ($365 B receipts vs $275 B outlays).

Which other month does the US usually run surpluses?  Other than April (when everyone pays their taxes).

Update: January usually has surpluses as well, at least in January 2018 ($51 billion) and January 2017 ($52 billion).  "Surpluses in January are boosted by the shift of certain payments, ordinarily made on the first of the month, from January to December because January 1 is a holiday."