33% can hold it off by seniority. 33% run into legality and work some or part, but end on Christmas or are required to be off. 33% work solid. 1% will have a random pairing ending super early and being off and home. For the last 10 years, the math has worked true, and in Pbs for Dec 2016 at my carrier, it still remains true overall.

On any given day, 60% of the workforce is working. Holidays get interesting, but overall, that's what it takes and it normally lines up with min days off for a ratio. Some are at min days, some pickup, then vacations, training, leaves, etc. 2/3 of the group will always need to work for things to work.