Function to Format Month Display Names

Hi, I'm using the following expression to return the month prior to today, which returns a number: 

month(edate(today(),-1))

While this works fine, I now want to format that number into the name of the month and not just the number - ie 1 is displayed as January, 2 is February etc. I created a constant to hold all the Month names but I'm mentally stuck at how to apply this in my interface (as part of a section label).

I'm looking for the label to be something like "Attestation for February" (where the month displayed is last month based on today's date). 

Thanks for any help!