Mothers' or Mother's??

In my post below about the Pilot Class Tea, I chose to use the plural possessive: Mothers'. (I know I should probably put quotation marks around Mothers', but it becomes hard to see the apostrophe!). I understand that the common spelling for "Mothers' Day" is "Mother's Day," but I cannot accept that spelling since, to my knowledge, there are tens of millions of mothers in the United States, making it one of the easiest singular vs. plural possessive decisions one will encounter! I will change it to the singular possessive only under the direction of one the following Calvert teachers: Jim Coady, current 10th Age boys' (note the plural possessive) homeroom teacher, 41 years at Calvert; Judy Rice, spelling, grammar, and composition guru from 1978-2008; or Betsy Cissel, who roamed the Lower School halls from 1955-1986 dispensing her "tough love" approach to grammar, writing, and all things Calvert. Until then, I shall use the following spelling: Mothers' Day!