I wrote a blog post quite a while ago on displaying GA data in the same format as Omniture. http://reports4u.co.uk/2011/07/10/omniture-visits-report-in-tableau-part-1/ I think this is a little more complex than what you're attempting but might be useful for you. The good thing about the Omniture visits report is that it compares with 4 weeks prior so the same week day is being compared.
You should be able to adapt the '4 weeks prior' calculated field to show the prior time period you want. Hopefully you understand the concepts explained in the 2 posts (part 1 and part 2) but if you need help modifying them for your specific case let me know.