There are actually 2 patch days, the second tuesday of the month (security and feature update CU) and one at the 3rd tuesday (or thursday), security only. But, yes. for at least 1809, it can be any day (because of the IP status of the updates)..
A long time ago I wrote a comment about builds released on a Monday: https://forums.mydigitallife.net/th...current-rs4_release.76695/page-5#post-1420744
the exception proves the rule It was just 3 times in 3.5 years, let's expect one tonight, based on those numbers.
No matter how many times it happened, you said that you had never seen a build released on a Monday (and in another comment you had asked about it), and I don't expect a build today, I just wanted to clarify that several builds have been released on a Monday.
I ran windows update and it found nothing but now I have a red sign on it and a tray icon saying " your device is missing important updates.." , timebomb?
My defender was updated on the same day when the build 18290 came. https://forums.mydigitallife.net/th...ip-fast-cur-rs_prerelease.78472/#post-1486387
People having that red update warning and not seeing an update in WU (defender platform, definitions or engine updates), did they modify the install in any way?