On April Fools a couple years ago while en-route I told all the pax that fog had unexpectedly formed at our destination and that ATC had advised us we would be holding for at least 3 hours (it was supposed to be a total of a 45 minute flight). The captain, in on the joke, came on about 2 minutes later and told everyone that is was an April fools and that we were in fact going to be 5 minutes early and the weather was perfect, as predicted. Everyone getting off was laughing, one person said "wow, you really got us good, nice job." While I think this one was probably taken a little too far...I think firing the guy would be waaaaaaaaaay too harsh. People need to lighten up...