I have a field that shows the last month's name, year on the same layout as my monthly calendar.
My calulcation is this:
Let( [ now = (CalFilterDateGlob ) ;
prev = Date ( Month(now) - 1 ; Day(now) ; Year(now) ) ] ;
MonthName ( prev ) & "," & " " & Year (now)
)

Now with the year change, this calculation is not working and i'm not sure how to get it to work with the year changing as you flip through the calendar.

Replace you last instance of "now" with "prev" like this:

Let( [ now = (Get(Currentdate) ) ;
prev = Date ( Month(now) - 1 ; Day(now) ; Year(now) ) ] ;
MonthName ( prev ) & "," & " " & Year (prev)
)