Many happy returns
Have many more happy days, especially birthdays.
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. The full version of the expression is 'Happy birthday and many happy returns of the day.'.
The first record I can find of it in print is in Joseph Addison's political discourse The Freeholder, 1752:
"THE usual salutation to a man upon his birth day among the ancient Romans was Multos et Foelices; in which they wished him many happy returns of it."
It has been suggested that the returns being wished for are the returns on an investement and that the salutation is wishing for someone's good fortune in business. The citation from Addison above makes it clear that isn't the case.