There are a couple of ways to achieve this, depending on the setup of your date dimension. Have a look at the YTD and PERIODSTODATE functions:

http://msdn.microsoft.com/en-us/library/ms146039.aspx

http://msdn.microsoft.com/en-us/library/ms144925.aspx

This is a common use for MDX so if you google it you will find absolutely loads of examples.