Hmm, I suspect there are a few Imperial days (like the Barrayaran's Emperor's Birthday) that everybody celebrates no matter what, and then there are separate Ghem and Haut holidays (and the non-aristocratic conquered peoples probably continue to celebrate their old holidays with the Ghem and Imperial holidays over the top.)

I suspect the Ghem holidays are all about military heroes and victories, and probably vary by satrapy. The Haut holidays are probably around breeding; we've seen that the days the Star Creche delivers the new children are important, and probably the days when they're concieved or registered, etc., are pretty major celebrations too.