The day Microsoft releases new patches (updates, fixes) for Windows and Microsoft applications. Also called "Black Tuesday," Patch Tuesday was introduced in 2003 to provide a routine schedule that network administrators could plan on.
Second and Fourth Tuesdays
On the second Tuesday of the month, Microsoft releases the most critical updates; however, updates that are not security related come out the fourth Tuesday of the month. In addition, Microsoft will release an "out-of-band" patch in between Patch Tuesdays if a very serious vulnerability has been fixed.
Over the years, some patches created problems, including causing applications or Windows itself to lock up. Pundits caution people to disable automatic Windows updating and wait a couple weeks after each announcement to learn if problems arose with the last patch. Needless to say, the vast majority of users choose automatic updating. See patch
and Windows as a service