bon anniversaire

When writing or saying "happy birthday" in French, the correct phrase is "bon anniversaire" and not "bonne anniversaire".

Here’s why:

• The word "anniversaire" is a masculine noun, so the adjective "bon" agrees with it in gender and number. Therefore, it remains "bon" even though it might sound like "bonne" when pronounced due to the liaison between the 'n' and the 'a'1.

For example:

• "Bon anniversaire !" (singular)
• "Bons anniversaires !" (plural, though "bon anniversaire" is more commonly used)1.

You can also use an alternative phrase, "Joyeux anniversaire !", which is equally correct and often used1.