I used to have one friend from Dubai who could never remember when April Fools was celebrated. He once came to us sometime near Christmas, played his prank, and called "April Fools!" when he was somewhat satisfied that we had been cleverly duped. Sometimes, we found ourselves having to fake it just to get him off it. And every time he did this, there would be one patient gem among us who would explain to him that April Fools was supposed to be called only in April, and only on the first day of the month, and never on other days, because then the fun was gone. He would then sit tight for a moment, ponder rather deeply for a moment or two, and come back to us yelling "Belated April Fools!" I got ya now! I tried to teach him a lesson several times by wishing him happy birthday sometime late in October or November when his birthday was actually in January.
But what aches the most is when the prankster in question plays the trick and forgets to call "April Fools" just in time.