considering the year...
=IF(DATE(YEAR(D2),MONTH(D2),)=DATE(YEAR(TODAY()),MONTH(TODAY())-1,),"A",IF(DATE(YEAR(D2),MONTH(D2),)=DATE(YEAR(TODAY()),MONTH(TODAY())-2,),"B",IF(DATE(YEAR(D2),MONTH(D2),)=DATE(YEAR(TODAY()),MONTH(TODAY())-3,),"C","D")))
mfd1979