Tuesday Jan 22 through Thursday 24, 2013 is the current planned time frame.
"Engineering teams have been preparing for the migration to minimize inconvenience to our users, but major service disruption is still expected during the transition. Our sites will be in read-only mode for some time, and may be intermittently inaccessible. Users are advised to be patient during those interruptions, and share information in case of continued outage or loss of functionality.
The current target windows for the migration are January 22nd, 23rd and 24th, 2013, from 17:00 to 01:00 UTC (see other timezones on timeanddate.com)."
Makes me wish I had Wikipedia already downloaded for offline access, it is on my to do list, have blogged about it before http://cliffsesportcorner.blogspot.com/2012/09/random-wikipedia-of-day.html.
Link to Download page http://en.wikipedia.org/wiki/Wikipedia:Database_download