Really interesting but how do I compare

Really interesting but how do I compare same day last year ? Let me explain. My Client has a requirement where they compare same day last year i.e. Monday with Monday, Tues with Tues !!! I know that the day will not be the same! eg. the want to compare Thursday, 1 Jul 2010 = Thursday, 2 Jul 2010 so ideally it is Date - 364 !! Any ideas?