Depends on whether or not they can get along.
My folks did my birthday, my mom's birthday, and Mothers' Day together, sometimes Christmas and Thanksgiving (we also did separate stuff).
I ate it up with a spoon. I loved the "together" celebrations, and I loved the "Daddy's people" and the "Mom's people" celebrations too. I think I loved the "Daddy's people" celebrations best, though-- and they would not have been the same if my mom had been there. It would have been awkward-- my mom hated my dad's sister with a purple jealous passion.
But my parents, for all they sucked at being married, were really really good at being divorced.
Not everyone can do that. Better separate celebrations than fighting, snide digging, and (worst of all IMO) forced-polite tension.
Double on the latter if you're talking about a spectrum kid.
That would have been AWFUL!! !!
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"