To compare attendance between two date ranges, go to Reporting>Reports>Attendance>Attendance Comparison.
This report will allow you to choose 2 date ranges and then show you a break down of each date range, as well as the difference between the two.
You also have the Historic Attendance Comparison report in this same area - Reporting>Reports>Attendance>Attendance Comparison.
Where you are able to create a report showing different groups.