Unless I actually know the person I'm talking to doesn't celebrate Christmas I go with "Merry Christmas." If you're at work talking to members of the public you have to be a bit more careful not to offend people but as a private citizen I'm going to go ahead and say it.
The axial tilt of the earth is the reason for the season.
Seriously though, Christians co-opted existing pagan festivals to hold a celebration at this time of year, which isn't when Jesus was born (no way would shepherds be out on the hillside with their sheep at night in the winter, nor would a man take his pregnant wife hundreds of miles along unlit deserted roads in the cold at that time of year).
Yes, that's what the celebration of Christmas is about, but it doesn't make a whole lot of sense when Christians get annoyed if people acknowledge that there are other festivals at this time of year, most of which predate Christmas.
Depends on who I'm sending the card to - but I even now wish them a happy holiday, because xmas is in the holidays (school) and most people get a holiday from work. I do say Merry Christmas. But I also wished everyone a happy Lunar New Year this year...so I cover a heap of bases...
I say Merry Christmas, to everyone, regardless of what culture, nationality or religion. How or why anyone would be offended by my saying Merry Christmas to them, I can't imagine. Can't understand where they are coming from. I worked with a girl who said to me that she didn't "believe in Christmas" and therefore didn't want people saying Merry Christmas to her. I told her that I knew that already because it showed on her face all year round. Just more so in December. I admitted to her that I only wished her a Merry Christmas out of habit, but Merry Christmas was a long way from what I actually wanted to wish her.
Very definitely Merry Christmas. If I am greeting people at that time of the year I say Merry Christmas. If they don't celebrate Christmas, I don't mind. I would never assume that they were insulted, or thought that I should be spending my time sussing out what their religious beliefs are. It is up to them to accept my wishes of goodwill graciously. They wouldn't like the alternative.