It's always a good idea to look up a function in FileMaker help if it doesn't produce the results you expected.
Month returns the month number, 1, 2, 3, ... 12 of a date in a field of type date. I would guess that you have imported your date into a text field instead of a field of type date.
The MonthName function will return the name of the month--once you have a date in a field of type date instead of text in a text field, but you probably do not need this funciton. Once you have a date in a date field, you can use data formatting on the inspector's data tab to specify a date format that includes the name of the month.