That provides your formatted output but I realize I missed the requirement to subtract one day from the date. This sample expression includes that calculation as well:
Code:
=Format(DateAdd(DateInterval.Day, -1, CDate(Fields!YourFieldNameHere.Value)), "MMM yy")
One question not anwered by you is what happens on the first day of a month? Should the output return the previous month? So if given 2/1/16, do you expect Feb 16 or Jan 16? What about 1/1/16? Does that return Jan 16 or Dec 15?
Based on your initial post, the first day of each month will return the month/year for the previous month. Every other day will return the current month. If that is expected, you are good. If you want the first day of each month to also return the current month, then you don't want the DateAdd function in your expression and should use my expression posted in my first response.
Robert "Wizard" Johnson III
U.S. Military Vets MC
CSM, CSPO, MCPD, CCNA, CCDA, MCSA, CNA, Net+, A+, CHDP
Senior Database Developer