AlyssaS - 4:01 pm on Mar 8, 2013 (gmt 0)
I'm not seeing anything unusual. Are you sure the Analytics code wasn't updated/edited on that date?
That's a possibility - that they changed the reporting (perhaps the length of the cookie for how they define a return visit), so it's not a real loss of traffic, just a reporting issue.
Then again if it was a reporting issue every site would show this, instead of just on the one site that appears to have lost traffic (the other sites seem to be fine).