it's volunteering to give up you legal holidays for work. in some countries, working on legal holidays automatically qualifies for higher pay, but that aside, i think it is fair to ask for volunteers first in case there are people who don't have family to see over the holidays or are not even celebrating. in fact i get pretty annoyed to not get to work on days that i don't celebrate. i'd rather have different days off that are more meaningful to me. of course, if there are no volunteers of this kind, then other incentives are a good idea, as is a rotation maybe