Question

What does ¿Cuándo es el cumpleaños de tu mejor amigo(a)? mean and how do i respond to it.

Answer 1

When is your bestfriend's birthday ! Amiga: Girl
Amigo: Boy
& el cumpleanos de mi amigo(a) es en

Answer 2

It means "when is the birthday of your best friend?" amigo is for a guy friend and amiga is for a gal friend.
the way u respond is "El cumpleaños de mi mejor amigo (a) es ...."
and fill in the date.