The most recent Patch Tuesday was on May 14, 2013, which you can read more about here: Patch Tuesday: May 2013. The next Patch Tuesday is scheduled for June 11, 2013.
Note: Most Windows users will experience more of a Patch Wednesday because they're prompted to install, or notice the installation of, the updates downloaded via Windows Update on Tuesday night or Wednesday morning.
Some half-jokingly refer to the day after Patch Tuesday as Crash Wednesday, referring to the troubles that sometimes accompany a computer after the patches are installed.
Security related patches that are released on days other than Patch Tuesday are referred to as out-of-band security updates.
More About Patch Tuesday
I also have several troubleshooting guides and other help and information about Patch Tuesday here on my site: