It's worth remembering that not all women are mothers.
Sure, there are worse things than saying "Happy Mother's Day!" to a non-mother.
But it's a careless assumption. For some childless women, hearing that is painful, for reasons it doesn't take a genius to guess.
For other non-mothers, it's simply annoying to be reminded that there are many who think all women aspire to the same kinds of lives.
Another thing to remember Sunday is that not all mothers are good mothers. But perhaps you are aware of that.