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.
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.
Microsoft now refers to Patch Tuesday as Update Tuesday.
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.
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