A calculation to see the last date of a month is "1st date of next month - 1".
[ FileMaker will automatically figure out what month and year (if needed).]
1st date of next month: Date ( Month (Date_)+1; 1; Year ( Date_ ))
Last date of month: Date ( Month (Date_)+1; 1; Year ( Date_ )) - 1
1st date of month: Date ( Month (Date_); 1; Year ( Date_ ))
[ I just remembered, a rather odd calculation for the "last date of month": Date ( Month (Date_)+1; 0; Year ( Date_ )) ]
To know what the above is for the "current date", use: Get (CurrentDate)
[ instead of "Date_" above, and set its "Storage Options" to "Do not store" (etc.) ]
Enter FInd mode 
Set Field [YourTable::YourdateField ; Month ( Get ( CurrentDate ) ) & "/" & Year ( Get ( CurrentDate ) ) ]
Perform Find 
For this month, that will enter 12/2014 as search criteria and you'll get all records dated in the month of December 2014.