You can set up summary reports that report information based on the current found set. You can perform finds to control what records are in the current found set. You can sort those records to group them by common values--such as all records dated in the same week's date range and then use sub summary layout parts to compute and display totals, averages, etc based on those groups as well as "grand total" results shown at either the beginning or end of your report. The individual records can be listed or just the sub total entries.
See this link for a summary report tutorial: Creating Filemaker Pro summary reports--Tutorial
This is not your only option for setting up a report.
A technique I have used, I would have a daily record SDATE as the field for the date (sales date). I have a field for the YYYY.ww (the week of the year). I have a field for YYYY.mm for the month of the year. I wrote my reports based on these summary points.
To get a field with the same date for every day of the same month:
DateField - Day ( dateField ) + 1
To get a field with the same date for every day of the same week (assuming the week is Sunday thru Saturday):
DateField - DayOfWeek ( DateFIeld ) + 1
The first calculates the date of the first day of the same month.
The second computes the date of Sunday of the same week.
Since both are dates, you can still sort on them and the groups will be ordered in chronological order. On a report, you can use custom data formatting to show just the parts of the date that are relevant such as January, 2015 for the first calculation shown here.