Your tax status is based on your marriage status on December 31, not April 15 of the following year.

Thank you for you answer. I did not know about the end of the year deadline.

Personal tax returns are done on a calendar year basis.

In fact, even after you are married, if both spouses work, you might be better off by not claiming married which will result in less tax withheld.    Often when two incomes are added together and the withholding has been at married rates, the combined income results in a higher tax.  Since each employer withholds as if theirs is the only income couples that both work often find that they are under withheld by claiming married.

I agree with Mac.  I have seen a lot of couple that get married, change their W-4s, and wonder why they owe so much money when they file their taxes.  As Mac says, if both spouses work, I recommend keeping it at "Single".

You can fill out your W-4 anyway you like.  However, if it is filled out incorrectly (married vs. single), you may owe significantly more tax - and in a worst case scenario - an underpayment penalty, for doing so.