The essence of day one is a new beginning. You are starting a new project, and it only last 3 days. To kickstart it you are eating foods that you don't eat on the next 2 days.
Grapefruit is low in calories, so go ahead and eat a whole one. (I don't know what you would do with the other half - stick it in the fridge for three days?). Don't add sugar of course. Grapefruit has fiber and helps lower insulin. But this is the only day you will eat it.
This is the only day you will eat peanut butter. Peanut butter should be eaten sparingly, since it has both fat and carbs, but it is a good transition food and it is filling.
This is also the only day where you will eat a lot of toast. The plan calls for 1 slice at breakfast and 1 slice at lunch but I usually have 2 slices at breakfast.
So congratulations on the new start! No snacking allowed, and only drink water, coffee or tea. Let's do this!