Have you heard that some people are taking offense at being issued the "wrong" kind of season's greetings? For pity's sake, people, nothing to get all snippy about. It's not at all hard. Here's the short course:
If you know somebody is a Christian, say "Merry Christmas". If you know that they're Serbian, like my mom's side of the family, say "Khristos se rodi" (or "Joyeux Noël" or "Felíz Navidád" or whatever the appropriate ethnicity is). If you know they're Jewish, go with "Happy Hanukkah". If you know they're Wiccan, say "Blessed Samhain". If you know they celebrate Kwanzaa, say "Joyous Kwanzaa". If you know they celebrate Festivus, say "Happy Festivus".
And in all other cases (that is, when you don't know), go with "Happy Holidays" and you can't miss.