A fool's day

a good one i heard on April's fool

Friend1: hey, Happy birthday!

Friend 2: huh? today not my birthday.

Friend 1: It must be, it's april's fool day.

Friend2: ........ (fumes)

one i play on others every year

Me: Look the teacher's on the roof!!! (i pick a name of the most unlikely teacher available)
surprisingly it works all the time:)