On Patch Tuesday (the second Tuesday of every month) Microsoft generally releases a handful of patches for security vulnerabilities found in its software. Usually, these bugs are not being exploited widely before Patch Tuesday as little information on them would be available outside of Microsoft. However, by issuing a patch, Microsoft can't help but give the details on the vulnerability.
By analyzing the patches, cybercriminals can identify the vulnerability in question and develop exploits or attack methods for it. While their attacks won't work on patched Windows installations, they can still work on unpatched Windows machines or machines that simply haven't downloaded their updates yet.
For this reason, the day after Patch Tuesday there is a rise in the number of attacks against unpatched systems. This has been dubbed by some as Exploit Wednesday or Day Zero. Often, malicious parties will also hold off on exploiting bugs they have found in Windows software until after Patch Tuesday.