OK, so we just passed Mothers' Day and I was curious as to why the card manufacturers all seem to think that the holiday is about one mother, i.e., mother's day vs. all mothers, mothers' day?
Mother's Day began in 1908 and was promoted by a women's organizer named Anna Jarvis. It was founded in large part as a day for women to mourn those who had died in war and to work for peace.
See explanation of its naming below from a National Geographic online article.
"For Jarvis it was a day where you'd go home to spend time with your mother and thank her for all that she did," West Virginia Wesleyan's Antolini, who wrote "Memorializing Motherhood: Anna Jarvis and the Defense of Her Mother's Day" as her Ph.D. dissertation, said in a previous interview.
"It wasn't to celebrate all mothers. It was to celebrate the best mother you've ever known—your mother—as a son or a daughter." That's why Jarvis stressed the singular "Mother's Day," rather than the plural "Mothers' Day," Antolini explained.
We can blame President Wilson. He wrote the proclamation. https://upload.wikimedia.org/wikipedia/commons/thumb/2/24/President_Woodrow_Wilson%27s_Mother%27s_Day_Proclamation_of_May_9%2C_1914_%28Presidential_Proclamation_1268%29._-NARA-_299965.jpg/768px-President_Woodrow_Wilson%27s_Mother%27s_Day_Proclamation_of_May_9%2C_1914_%28Presidential_Proclamation_1268%29._-NARA-_299965.jpg
Regardless of the original intention of the creator and founder of Mother's/Mothers/Mothers'Day, the punctuation presently varies in use.
[I, personally, prefer the apostrophe after the "s", as I tend to celebrate all mothers and motherhood and nurturers of any gender on this day, whether or not they have birth- or adopted children, keeping in mind that some people have had more than one nurturer, eg, mother and step-mother, 2 adoptive mothers.]