www.yoursite.com/?m=200507&cat=5

where yoursite.com is your domain,
200507 is the month you want, and
5 is the category id for politics

cool! but can this be automated in the navigation bar?

Probably. I don't know how, but I'm sure the _archives.php or _categories.php file in your skin folder could be tweaked to give you the results you want. That's going to be a long navigation bar, though. If you have 4 categories and 12 months, you'll have 48 links in your sidebar.

Yes you're right. But I thought, maybe one could make it like this that one would be able to select month(s) and category the same way as multiple categories (via checkboxes)