Which is the biggest card sending event...new year, christmas, valentine's day ?

Thats hard to say I don't think many people send cards for new years more are sent for Christmas. If you mean being sent my regular mail I would have to say Christmas is bigger then Valentines day. I would think that Birthday would be bigger then valentines day also but not sure what you mean by sent. If you count all the cards that little kids give to each other for Valentines day and plus what others give to one another then yes it probably is valentines.