Some of you may have noticed, that date archive widget and the BlogTree pages do not work well together. Specifically the url fragment such as '/blog/date/2011/04' doesn't work if 'blog' is a BlogTree instance, but DOES work if it is a BlogHolder instance.
Inside BlogTree.php and the BlogTree_Controler class. You need to add 'date' to the $allowed_actions array.
Sounds like this should be fixed in the core download so make sure you either submit it as a pull request to the module's github page (https://github.com/silverstripe/silverstripe-blog) or at least as a ticket on open.silverstripe.org.
Actually, it seems that the github version has this simple fix already. But still the current download version has the bug. So I guess this is for people that are just downloading it from the mail module section on silverstripe.org, or have a version in production that exhibits this bug.