Change Month Number To Text (1 to JAN, 2 to FEB etc)

Hello, 

I have created a field that displays the Month an Opportunity Closed. It is coming over as a number but I would like this to be displayed as text i.e. 1 = JAN, 2 = FEB, 3 = MAR etc.

The Field I have created is month_closed_c (Text Field) and is using this formula: monthofyear($date_closed)

How can I convert the number value this field returns in to a text value?

Thanks