Is it rational for, say, Muslims to not celebrate Christmas?

[Duty]

I'm an atheist and still celebrate it as a secular holiday

When you say "celebrate it," what does this consist of? Just taking the day off? Giving presents? I assume you don't do any of the church stuff. Does your family do something special?

Here in Japan Christmas is barely noticed. For some reason there is a special kind of cake people eat for Christmas, and they're surprised that it's not an American custom. Also it's normal when you're dating to get an expensive hotel room and get laid on Christmas eve. Back when I was dating this was something to celebrate. 

New Year's is the big holiday here, and I "celebrate" it if that means staying home with family, like everybody else does. I mean, if people's work schedule shifts to align with everybody else, then you observe the holiday, I guess. But that's not quite the same as "celebrating." 

If it's a big enough deal where you are, you kind of have to include it in your life, at least a little.

"Celebrate" seems the more appropriate term than "observe" imo because the latter has no implication of fun/pleasure type stuff - quite the opposite. Our usual (non-covid) xmas involves enormous turkey roast, preceded by bacon and eggs for breakfast and smoked salmon + cream cheese blinis for 11 O'Clock ish snack with Carva. Crackers with lunch (large family gathering of multiple generations) then gift exchanges after lunch. Roast beef on boxing day....yup, a big deal, basically, and not so much as a whiff of religion.

I gather from the film "Silence" that Christianity was not, erm, "well received" in Japan, back in the day, so probs never took root and never underwent the "post religion transformation" into Santa's walletfest current prevalent incarnation. New years eve here is never as celebrated as Xmas I guess....everyone's partied out from Xmas I guess.

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ETA: here's a still from "Silence" - "refuse to renounce you Christianity? We've got a fix for that" type shenanigans.