0% drop in visits starting from March 6th

[pluis3103]

Hi,

One of my websites, that usually had around 1500-1800 visits a day, suffered a 50% drop in visits starting from March 6th, 2013.
No major changes to the site were done that day, despite a new banner (with a link to a Facebook app) and a new calendar with events (a basic calendar in php without javascript - html basically).
There haven't been any warnings in Google's Webmaster Tools either. Any idea of what's going on?
I appreciate the help.