if by 'their' you mean his/her, then:
"son anniversaire est le/l' [date]"
if by 'their' you mean a group of people:
"leur anniversaire est le/l' [date]
a birthday is 'un anniversaire' in French
Birthday is "anniversaire" (masc.) in French.
A birthday dinner is "un diner d'anniversaire" in French.
A birthday cake in French is 'un gâteau d'anniversaire'
anniversaire
"Today is my birthday!" in French is "Aujourd'hui est mon anniversaire !"
If you mean birthday date My birthday is_____-Mon anniversaire est____
my birthday
Anniversaire
yes
Anniversaire
d'anniversaire