I see a lot of people bring up part time schooling, with a couple days on or reduced time, but I haven't seen any threads about shifting holiday.
Basically in a year round system, there are Tracks. Each student is assigned a Track (and families can choose to stay on the same Track). The Tracks are divided (Track A, B, C, D) so that the students and teachers assigned to that specific Track are allocated holiday. It's two months off, one month off continually with the one month off shifting according to the Track.
For example
Track A goes to school January, February and is off in March, then goes back April, May, and is off in June, then goes again July, August, then off in September, and back in October, November and off in December.
Track B, C, D is the same but the month off shifts throughout the year so that it reduces the number of students. Holidays for Easter, Christmas, and New Year's are still given, but are shorter.
This might help with the space in buildings.