Hi,
you must have already created a parameter to enter months remember to restrict the user to enter the Months in fixed format or else the formula 2 will not work what you can do is create a DropDown List of Month Parameter which will be non editable.....
Then create a formula to get Month Name
Frm1
Monthname(month({Table.Datefield}))
Then create another formula to convert the entered Month in paramter to number.
Frm2 this checks what month name you have entered in parameter
Select {?My Parameter}
Case "January","Jan" :
01
Case "Feburary","Feb":
02
Case "March","Mar" :
03
Case "April","Apr":
04
Case "May":
05
Case "June","Jun":
06
Case "July" ,"Jul":
07
Case "August","Aug":
08
Case "September","Sep" :
09
Case "October", "Oct":
10
Case "November" ,"Nov":
11
Case "December","Dec":
12
Then in the record selection formula
Compare the following
month({Table.Datefield}) = Frm2-1
So if you enter June in parameter it will give records for May and so on.....
Cheers
Rahul