I would like to know if the verb 是 (shi) is optional?
你的生日是什么时候？ (Ni de shengri shi shen me shi hou?)
你的生日什么时候？ (Ni de shengri shen me shi hou?)
Which of those two sentence used more?
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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:
These are really colloquial way to speak
I'll take the last one when speaking :-)
In my ears, 你的生日什麼時候？ sounds more authentic than 你的生日是什麼時候？ Which of the phrasing is more common might depend on location though. As a non-native speaker, one should remember that nouns or pronouns can be predicates in Chinese. Sometimes non-native speakers are influenced by their native language and add 是 as copula, even though it is not necessary. Other examples of sentences, where nouns or pronouns are predicates:
他們中國人嗎？ (In this sentence 他們是中國人嗎？ would put emphasis on 'being Chinese' and increase the tone of doubt.)
'你的生日是什么时候？’seems more natural than '你的生日什么时候？' But it does not mean that '你的生日什么时候’cannot be used in certain situations. Otherwise, it seems to me that '你生日什么时候？' sounds more natural than '你的生日什么时候？’. I do not know exactly the reason for this. It is possible that this relate to the number of characters in one utterance.