Joyeuses mamans
Here's why:
* Joyeuses is the feminine plural form of "joyeux" which means "happy" or "merry".
* Maman is the French word for "mother", and its plural form is "mamans".
You can also use "Bonnes fêtes aux mamans" which translates to "Happy holidays to mothers". This would be more appropriate for a more formal setting or a holiday greeting.