With the LOOKUP formula (Kevin's), you need to put the months in alphabetical order, with a corresponding shift of the days, i.e.:

=LOOKUP(D1,{"Apr","Aug","Dec","Feb","Jan","Jul","Jun","Mar","May","Nov","Oct","Sep"},{30,31,31,28,31,31,30,31,31,30,31,30})

although this would always give 28 days for February and not adjust for leap years.

Hope this helps.

Pete