You could use this code in the bottom of your MTEntries tag:
CODE
<MTEntries>
=====ALL MAIN MTEntry Tags go here!===
<MTEntriesFooter>
<MTEntryPrevious>
<a href="<$MTEntryLink archive_type="Weekly"$>">Previously: <$MTEntryDate$></a>
</MTEntryPrevious>
</MTEntriesFooter>
</MTEntries>
Make sure that you have weekly archives turned on in the Blog Config under Archiving.
Hope that helps,
Kristine