This is probably where a custom date lookup table (date dimension) will be helpful. The exact structure and how you might use it would depend on the structure of the rest of your data. I've created a simple example using the SuperStore sales demo data provided with Tableau and a very simple date table that looks like this:
Date Holiday 3/31/2013 Easter 4/8/2012 Easter 4/24/2011 Easter 4/4/2010 Easter
Then I blended the data to create a comparison of Sales for the week of Easter for each year (2013 doesn't exist in the Sales data).
A quick table calculation gives the difference from the previous year.
Easter Week Sales.twbx.zip 84.5 KB