Last modified: 2011-11-29 03:20:58 UTC
At http://download.wikimedia.org/backup-index.html for the enwiki dump the date "2008-04-05 14:55:41" is shown, and at http://download.wikimedia.org/enwiki/ "2008-Apr-05" is mentioned. But both pages do link to http://download.wikimedia.org/enwiki/20080312/ instead! The 2008-Apr-05 versions seem to be non-existent, although mentioned!
Also with dewiki, by the way!
2008-04-05 14:55:41 is the last time at which that dump process updated.
2008-03-12 is the date on which it began.