Sand - 8:09 pm on Feb 4, 2013 (gmt 0)
Nothing significant happened on the 21st (if you look at Mozcast).
I've found that Mozcast is generally a really poor indicator of Panda refreshes -- they're often little more than a blip.
With that said, as someone who had a site impacted (positively) by this last refresh, it did start rolling out a few days before they announced it. I started to notice a difference on the Thursday evening before, and the refresh was announced on Monday. I think that it was already completed by the time that Google said it was rolling out (as usual). So matching up dates can be difficult sometimes.