'Tis the season to be pedantic, so who am I to fly in the face of tradition?
I have bought many packs of Christmas cards over the years, and they fall into two categories: Silly and Religious. This year is no exception, and I have some nice Nativity ones, which say "Merry Christmas" inside them, and some nice non-Nativity ones (geese, Miss Piggy and 1950s skaters, since you ask), which say "Season's greetings" or similar.
I am not particularly bothered about what Christmas cards say inside them - yes, we all know it's one of the Big Two of the Christian year, but I don't (as an agnostic myself) feel personally affronted if this is either pointed out to me or studiously (and with great PC) avoided.
After all, it's nice to think someone thought of you and sent you a card, whatever the feeling behind it.
HOWEVER. I bought some nice Robert Sabuda pop-up Christmas cards (well, I think they're nice, so that's the important thing), which combine some seriously Nativity pop-up images with some inoffensively snowman 'n' reindeer pop-up images (perfect for the rabid Zen Vegan in your life). And the one with the pop-up The Baby Jesus In A Manger says "Happy Holidays" in it.
Let's be honest, the sentiment "Happy Holidays" is designed to show Those Of Other (Or No) Faiths that while you wish them a nice break over the December 24th - January 2nd period, you wouldn't dream of trying to convert them at swordpoint like your ancestors might have done. Affixing this to a picture of The Baby Jesus is as pointless as asking people to come to church with you for a coffee. I appreciate the strenuous efforts the manufacturers were making to remain politically correct (and I am, as I may have already said, a fervent agnostic), but there are limits.