kittya , Christmas is as big a deal in the States as it is in the UK and Ireland, but Thanksgiving is also a very big holiday, and for some families might be the one time each year when everyone makes an effort to be together. The day before Thanksgiving is always the busiest travelling day of the year, so I guess that says something.
Christmas definitely has the edge in terms of religious significance (obviously), commercial hype, excitement for the kids (there are no presents at Thanksgiving!) and months-long build-up, but in terms of family traditions and reunions the two days are about equal, in my experience (i.e. in my family and among the people I've known). Many families will spend Thanksgiving with one set of in-laws, then Christmas with the other, and swap the following year.
Thanksgiving always falls on the fourth Thursday of November, so the typical time off work / school would be Thursday, Friday and the weekend. That depends on the job of course and sadly the whole "Black Friday" thing has kind of taken over to the degree that anyone working in retail would likely have to go in on the Friday.
If I were to refer to the "holiday season" I would mean the period of time starting with Thanksgiving, through Christmas and up until New Year's Day. I think most Americans would mean the same thing (substitute for Christmas Hannukah, winterval, or whatever if you wish).