My birthday is in/on May 16th.
on
Stella's birthday is in/on December.
in
Stella's birthday is in/on 10th December.
on
Stella's birthday is in/on Saturday.
on
This year, my birthday is in/on Tuesday.
on
My birthday is in/on May.
in
My birthday is in/on the spring.
in
Stella's birthday is in/on the winter.
in