If you are able to edit the SQL then do as robert says. If you aren't good with SQL or are unable to edit the SQL for some reason then here's an idea:

If the week field is a text datatype (not a true date) then you could create a calculated column as

=Left(Fields!Week.Value,4)

This assumes that your weeks are as you've shown in your example, as either 3 or 4 letters followed by the day of the month.

You will also want to include the year in the group, that's important if your report spans multiple years.

If the week field is a proper date datatype then you can use this formula:

=MonthName(Month(Fields!Week.Value),False)

If you add a parent group to the details row, use that calculated column as the group.

You can then add totals for the groups. If you right click on the $Sales_Item1 details cell it will give you the option to "add total".

The results will look somewhat like this

Month Week $Sales_Item1

Aug Aug 20 1000

Aug 27 500

Total 1500

Sept Sept 03 2000

Sep 10 100

Sep 17 0

Sep 24 0

Total 2100

If you really want it as per your example with the totals per month separate, then you can add a second tablix below and just use the grouping in it, i.e. change the details group to be grouped by month (see formulas above) and use something like =sum(Field!@Sales_item1.value) for each cell expression.