I.e. someone who is just altogether awesome and you want to associate yourself with for that particular day.
Kind of like Mother's Day or Father's Day,
except 293487234678913465 times better and funner.
Also a Facebook application that chooses one of your friends at random (and tends to be rather accurate).
robot #1: HEY can i be your FOTD on facebook?
robot #2: i'll only post you as my FOTD if you actually are :)