Do I have to include 是 in 你的生日(是)什么时候？
你的生日是什麼時候?
(When is your birthday?) is the proper way to propose a question.
你(的)生日什麼時候? is gramatically incorrect but seldom heard when speaking for shortness

Also, 
你(的)生日是哪一天?
(What day is your birthday?)
and, 你什麼時候生日?
are frequently used for asking about birthdays.

These can also be used in many other occations. For example:


你什麼時候出去的? (When did you get out?)
你們什麼時候要走(=離開,出發)? (When are you guys going to leave?)


These are really colloquial way to speak

I'll take the last one when speaking :-)