I have a theory about holiday wishes. People celebrate a lot of different holidays, and especially with people you don't know well it can be hard to keep them straight. Or even more with strangers.
Trying not to wish somebody a happy whatever if they don't celebrate it results in less well-wishes, and I think there should be more. After all, even if you don't celebrate it you still go through the time and it would be best if it were happy. So while it's great if you can wish someone a happy thing that they celebrate, don't let it stop you wishing them a happy thing that you celebrate.
So, even if you don't celebrate U.S. Thanksgiving, have a happy Thanksgiving day anyway!