I celebrate Christmas. So I say Merry Christmas--to those I know who also celebrate Christmas. Otherwise, I say Happy Holidays. I feel that's inclusive. There are several holidays in December, and of course there is New's Day in January. So Happy Holidays can cover a lot of ground, especially if you are saying it to someone early in the month who you probably won't see again for another month or so.
Store clerks are probably instructed to say Happy Holidays, because they don't usually know what holidays their customers might celebrate. And, of course, the customers might not know what, if any, holidays the store workers celebrate. So again, Happy Holidays covers it all.
Since already I'm seeing ads for Christmas on TV and in newspapers and magazines, and Christmas music will be taking over some of the radio stations either just before or just after Thanksgiving, depending on the station, I think there's plenty of Christmas in the air.
And of course decorations will be showing up in stores, churches, homes and in many businesses. The businesses may have more generic decorations, such as snow scenes, since, again, not everyone working at or coming into the business will necessarily be celebrating Christmas. But there will be Santas and Christmas villages and angels and Nativity scenes in many places.
So, it's early, but let me wish you Merry Christmas or Happy Holidays--whichever works for you!