Holiday has several meanings, and in the U.S. in particular, to offer happy holidays is a way to send non-religious good wishes for this particular time of year. It's not wishing someone a good vacation. You can send it to christian and jewish and other religion friends as well as the non-religious.

"Happy Holidays" acknowledges the December seasonal array of events and is perfectly valid, though purists within their particular industrial religions will protest that there is a "reason for the season" that they are privy to and try to appropriate the season or dates, ignoring myriad autochthonous religions and the historic harvest and solstice celebrations from around the world.

Your mileage may vary.

SRS