Those number are total days over a 4 month trimester. So 52 days = 13 days/month 60 is 15 days, 72 is 18 days and 76 is 19 per month. They can add or subtract 2 days per month but the total for the quarter has to be less than or equal to 52, 60, 72 or 76. Schedules are bid 4 months in advance so you aren't stuck on one schedule the whole year.