I think it depends upon the teacher.
My youngest dd's kindergarten class celebrated the major Christian, Muslim and Jewish holidays, and Kwanza as well. They just learned a few songs, the history of the holiday, and how it is celebrated in the US today.
In dd's classes, typically there has been either no religious instruction, or just every once in a while, with a major holiday. Typically Christmas, Chanuka (spelling, sorry..), Ramadan, and Kwanza. The history of the holiday and the way it is celebrated today is typially how it is presented.
We are in a comparatively liberal town in an otherwise EXTREMELY conservative state on the edge of the bible belt.