This document shows how you can use Excel and Tableau to blend a fiscal calendar into your visualizations, without having to alter your raw data.
All you need is to create your fiscal calendar in an Excel worksheet, then blend it into your data set. You are welcome to copy the 2012 4-4-5 Calendar from the attached workbook.
Create your fiscal calendar in Excel with a Column for Actual Date.
Connect to the workbook in your Tableau workbook.
In the Top Menu, Select "Data" -> "Edit Relationships".
Create a relationship between "Actual Date" in your Fiscal Calendar and "Date" in your original data set.
Now, you can slice your data using your Fiscal Calendar instead of Tableau's built-in Calendar.