In my last job, I used to be the one in charge of acquiring, testing, piloting, and deploying the Microsoft Patches to over 8,000 workstations and 800 servers in 30 countries worldwide. We did not use Microsoft's product, but one developed by Novadigm called Radia Patch Manger (later acquired by HP, and the name changed to just Patch Manger). So, I got to know very well all the ins and outs of patching.
Like clockwork, Microsoft will release its "monthly" security patches and updates (the ones that are named like MS09-068) on the 2nd Tuesday of every month. As far as what time, I did not specifically keep track, but they generally would be available on the website to view all the details around 12 noon CST (so that would translate to 10 AM PST).
Of course, Microsoft has been know to release "out of band" or "out of cycle" patches, and these can come at any time.
So, just remember 2nd Tuesday of every month a little after 10 AM PST, and you'll be fine!
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