... was e-mailing a note to a facebook friend on his birthday to wish him many happy returns of the day, and suddenly wondered what that actually means. i've not heard it here in the states, but it used to be around quite a bit i think when i was growing up in london. anyway, wiki and a couple of other googled sites have this to say:
"Many Happy Returns is a greeting, usually reserved for birthdays. The term itself refers to the passing year. Since the 18th century this has been used as a salutation to offer the hope that a happy day being marked would recur many more times. It is now primarily used on birthdays; prior to the mid 19th century it was used more generally, at any celebratory or festive event.
Current usage is often as a more formal option than 'Happy Birthday'. It is also often to be found on greetings cards.
Its earliest attributable use was by Lady Newdigate in a letter written in 1789 (and published in Newdigate-Newdegate Cheverels in 1898): "Many happy returns of ye day to us my Dr Love.""