=(ПОИСК(A1;"janfebmaraprmayjunjulaugsepoctnovdec")+2)/3

=(ПОИСК(ПСТР(A1;2;2);"anebarprayunulugepctovec")+1)/2

Sub Month()
       Dim m As Range
       Set m = Range("a1")
           Select Case m
           Case "Jan"
           MsgBox 1
           Case "Feb"
           MsgBox 2
           Case "Mar"
           MsgBox 3
           Case "Apr"
           MsgBox 4
           Case "May"
           MsgBox 5
           Case "Jun"
           MsgBox 6
           Case "Jul"
           MsgBox 7
           Case "Aug"
           MsgBox 8
           Case "Sen"
           MsgBox 9
           Case "Oct"
           MsgBox 10
           Case "Nov"
           MsgBox 11
           Case "Dec"
           MsgBox 12
           Case Else
           MsgBox "Нет такого месяца"
           End Select
End Sub