I had problems with the Holidays calendar, too. In my situation, the Holidays calendar was still listed and checked, but the events didn't show up on the calendar display.
I think the problem (for me) was that iOS wasn't getting the data. I went to Settings>Mail, Contact, Calendars>Fetch New Data>Holiday Calendar and switched it from Fetch to Manual and then back to Fetch.
Then I went back into the Calendar app, tapped Calendars at the bottom, and pulled the screen down to force a refresh of the data. After the little circle stopped spinning, the Holidays showed up again.