These are 3 accepted ways to write out that special greeting to you mother and/or other mothers. It really comes down to its intended use. Mothers Day without the apostrophe describes a day for Mothers not a day belonging to Mothers. Mother’s Day with an apostrophe before the “s” means that the day belongs to one specific Mother – it is important to note that this version was the original campaigner for creating Mother’s day and specifically wanted the apostrophe to be before the “s”. There is a 3rd version written out as Mothers’ Day which is meant to be written if the day is shared among all Mothers.
Simply put if I were addressing one mother such as my own then I would use the original version – Happy Mother’s Day. If I were addressing many mothers then I would probably use Happy Mothers’ Day. I’m not really sure how I feel using Mothers Day without any apostrophe… I can see myself using this version on my calendar and using it in a descriptive manner instead of using it to greet my mother or a group of mothers i.e. “I’m going to visit my mom for Mothers Day”.
How do you write it out and why?
Further Reading: http://www.history.com/topics/holidays/mothers-day