I would probably do it like this:

In summary!A2: have a data validation drop down of the months, linked to your Sales sheet row 1
In summary!B2: have a data validation drop down of the months, again linked to your sales sheet row 1
These would become your start and end dates. You can then use this formula in B3:

In A3:A8 have your geography headings, then in b3:
then in b3:
=SUM(INDIRECT(("Sales!"&ADDRESS(MATCH($A3,Sales!A$1:A$7,0),MATCH(Summary!A$2,Sales!A$1:V$1,0))&":"&ADDRESS(MATCH($A3,Sales!A$1:A$7,0),MATCH(Summary!B$2,Sales!$A$1:$V$1,0)))))
and copy down.