The modern holiday of Mother's Day was first celebrated in 1908, when Anna Jarvis held a memorial for her mother in America. She then began a campaign to make "Mother's Day" a recognized holiday in the United States. She was specific about the location of the apostrophe; it was to be a singular possessive, for each family to honour their mother, not a plural possessive commemorating all mothers in the world. This is also the spelling used by U.S. President Woodrow Wilson [in 1914] in the law making official the holiday in the United States.
Have a happy Mother's Day, Mom!
Thanks to the Grammar Girl for her tip on the proper spelling for the day today.