From my experience, most of the salt on Mother's Day and Father's Day comes from people who had bad relationships with theirs. They will attempt to somehow spoil the day for others, who have privilege or something. They usually do nothing more than make shitty comments on someone's FB post about how fun it is hanging out with dad on This Special Day.

I've occasionally seen people ask "what about non-binary parents?" and things like that. These amazingly outside-the-box people who are unbound by society's conventions are unable to come up with their own holiday.