Another solution, example as follows.

1. In B2:B13 put the month  names, i.e. January, February, ...
2. Name these 12 cells e.g. MonthNames (select the 12 cells, Insert,
Name, Define, type in MonthNames).
3. In C2 type in =MONTH("01-" & MonthNames & "-06") and hit enter.
4. Select the 12 cells C2:C13. Hit F2 in cell  C2. Hit control-shift-
enter to create an array.
5. Select the 24 cells B2:C13. Name them MonthNumbers.
6. In cell D2 put whatever Month Name is being looked up, e.g. June.
7. In cell E2 type VLOOKUP(D2, MonthNumbers,2,False).

This solution will return the MonthNumber in E2, and does not
require that the MonthNames array be sorted. HTH.