Patch Tuesday is the name given to the day each month that Microsoft releases security and other patches for their operating systems and other software.
Patch Tuesday is always the second Tuesday of each month and more recently is being referred to as Update Tuesday.
Non-security updates to Microsoft Office tend to occur on the first Tuesday of each month and firmware updates for Microsoft's Surface devices on the third Tuesday of every month.
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.