TBPL: Mother's Day seems fine to me if it is the day I give a gift to my mother, however, if I accept that everyone celebrates on the same day then Mothers' Day would be correct. In either case an apostrophe is required. Of course, as an editor you may wish to save on ink, or column inches, by omitting as many apostrophes as possible.
Of course it has been commercialized over time. I remember it as 'Mothering Sunday' which was as much about giving thanks for 'Mother Church'.

Nigel