This year, I was looking for information on the history of the holiday (at my son's request) and found this:
In 1912, Anna Jarvis trademarked the phrases "second Sunday in May" and "Mother's Day", and created the Mother's Day International Association.
"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 the law making official the holiday in the U.S., by the U.S. Congress on bills, and by other U.S. presidents on their declarations.
Common usage in English language also dictates that the ostensibly singular possessive "Mother's Day" is the preferred spelling, although "Mothers' Day" (plural possessive) is not unheard of.
Source: this Wikipedia page.
So, Happy Mother's Day to you. I know US Mother's Day isn't until tomorrow, but my son already gave me his gifts, since he couldn't wait.