"Second Sunday In May"

One hundred years ago, Anna Jarvis trademarked the phrases "second Sunday in May" and "Mother's Day".   She was specific about the location of the apostrophe; it was to be a singular possessive, for each family to honor 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 United States.  No matter how you spell it, it's the sentiment that matters on this special day.