Prepositions in questions

A question from Lucy in Taiwan:

Which of the following are correct?

1. What day is your birthday?
2. On what day is your birthday?
3. What day is the Christmas party?
4. On what day is the Christmas party?
Thanks!


Hello Lucy. Thank you for your question about the use of prepositions, words such as ‘on’, ‘in’ and ‘at’, in questions.
The brief answer to your question is that both styles of question, What day is your birthday? and, On what day is your birthday?, are possible and both forms are commonly used. However, there is a slight difference between the use of these forms; a difference which is connected to formality.


If we want to use very formal grammar, we say On what day is your birthday?. We say this because the answer, which should be It is on Tuesday, includes the preposition ‘on’.


However, this is an example of a grammar rule that is not very strict, because, in everyday English, in less formal situations, most people say What day is your birthday? Leaving out the preposition is a very common and well-accepted style.


You will also hear What day is your birthday on? Again, this is a less formal style, but it is commonly used.
To finish, both of your questions concern, in a sense, the time of an event and, in that case, it is very common to use ‘when’ rather than ‘what’. For example,


When is the Christmas party?
instead of
On what day is the Christmas party?
or
When did Gordon Brown become Prime Minister? 
instead of
In what year did Gordon Brown become Prime Minister?


I hope that makes things clearer for you, and I recommend that you listen carefully for the different styles when listening to people speaking English, perhaps on the radio or in films, so that you can get a feeling for these differences in formality.