It is very American-like to be “very happy for you” and “wish you well” and “wish you all the best” on any possible occasion. By nature Americans, as a nation, are open towards foreigners. “We are a nation of immigrants; we all came here from somewhere else”. According to popular belief, even the native Indians, historically, came from Asia.
This general attitude of friendliness carries over to celebrating holidays. American Holidays such as New Year's Day, Independence Day, and Thanksgiving are, of course, followed by everybody. On top of that there are a number of special days and occasions: ethnic, religious, a.s.o. (and so on) that, even though observed only by limited groups of people, have become a part of American tradition and culture.