A quick, easy solution (if your info is organized like you described) is to create a pivot table and let it do the work for you.
- Select your data (columns A:C) and choose Insert > Table.
- Create a Pivot Table from your data table (step 1).
- Add your Date Assigned field to the Row Labels box in the Pivot Table Field List.
- Right-click on a date in the Pivot Table and choose the Group option.
- In the Grouping Dialog Box, choose the level of grouping you want. To create weeks, you'll need to select Day, then set Number of Days to 7.
- Finally, add your summary data to the Values Box. Here you can choose to have your data in any format, including summarizing by Average, Min, Max, or Running totals-pretty much any aggregation you can think of.
The advantage of using the Table for your data is two-fold: first, any custom columns you add to the table auto-calc for the whole table and are also usable in the Pivot. Second, whenever you update your Data Table, the Pivot Table automatically updates with the new data (or changes to the existing).
EDIT: Instead of using the Pivot Table's grouping (which is less than ideal without an OLAP data source), you can easily modify your existing table to improve performance. In addition to your original three columns, you can add columns for any other date period (e.g. year, month, week) that calculates off of your intial date value. Just add these to your Pivot Table in order and you can roll up or down to the level of detail you'd like.
- Column D =Year([@DateAssigned])
- Column E =Month([@DateAssigned])
- Column F =WeekNum([@DateAssigned])
- Column G =Day([@DateAssigned])