"Riddle Me This!"

A man's birthday is Dec. 15. He wakes up on what seems a perfectly ordinary workday in the middle of May. For some reason, though, his wife and his children wish him happy birthday before he leaves. He finds this more than a little curious but doesn't worry too much until he goes into work and finds that all his co-workers have covered his desk with birthday cards and multitudinously repeat the sentiment. Even more curiously, his parents call later that night and wish him a happy birthday as well. Putting aside the notion that this is one huge practical joke, why are they all doing this?