But...is it bad to be progressive?
It gives children raised in a Christian society more context to observe another prominent religion outside of the lens of jihadism that the news tends to focus on. I think that's progressive in a real way. I can't imagine it matters much to competing the curriculum either.
How about we teach people critical thinking rather than passive aggressively trying to make them accept other things by taking school days off for a group that probably doesn't make up more than a few percentage points of the wider audience? How about we teach about all religions inside of history/social studies in a secular way, that goes over things like the Crusades to give people a basis to compare and contrast... Shit, we already do that. Observing a few holidays isn't going to undo parents, culture, and hundreds of other effects. It's a tiny thing that only will bring more scorn and dislike than it will bring acceptance or progress. And if the school does already have a significant portion of people who do believe in the Islamic faith, then the students will get exposure to it by that basis alone, no holidays needed (although you should accommodate and have days off at that point).