Formula to calculte previous month name from current month selected

Morning all

I have a data validation drop down which is by month.

What I am looking to do is have a formula that once a month is selected from D5 to show the previous month name in D20

Is this possible so no matter what month you select in the drop down of D5 below automatically in D20 the previous months name will be shown.

I have attached an example, cheers