I'm sure most people know that Microsoft designates the second Tuesday of each month as "Patch Tuesday" - the standard day for releasing all kinds of updates (both security and non-security) to Windows and other Microsoft products and technologies. Marking this in Windows Calendar is a trivial operation:
However, less well known is that on the Thursday before Patch Tuesday, Microsoft release an "Advance Notification" which indicates how many security patches are planned for release on the following Tuesday, what products they affect, and their severity. But how would you add that to Windows Calendar?
You can't do it via the GUI - it's simply too complex a rule. Wait, how would you describe it in any calendar anyway? Windows Calendar, like the majority of calendaring applications, uses the iCalendar format (RFC 2445). You can't specify that an event is positioned relative to another event, but iCalendar is still very flexible, as it allows you to combine restrictions on a repetition rule (see section 4.3.10 of the RFC for the gory details).
The simplest way I found to get the correct Thursday is to restrict the event using two criteria: month day must be 3 - 9 inclusive and it must be a Thursday. This works because only one day in the range 3 - 9 can be a Thursday each month, and it is five days ahead of the range of possible Patch Tuesday days (8 - 14). The rule is thus: "RRULE:FREQ=MONTHLY;BYMONTHDAY=3,4,5,6,7,8,9;BYDAY=TH".
To add such an event:
- Start by adding a normal monthly repeating event and give it a unique summary.
- Close Windows Calendar.
- Find all the calendars in C:\Users\<username>\AppData\Local\Microsoft\Windows Calendar\Calendars.
- Open the correct calendar file (they use the name set in the GUI).
- Find the correct event by searching for the summary of the event.
- Just above the summary is the "RRULE", which should be replaced with "RRULE:FREQ=MONTHLY;BYMONTHDAY=3,4,5,6,7,8,9;BYDAY=TH".
- Re-open Windows Calendar, and it should appear on the Thursday before the 2nd Tuesday of every month.
Done! You can even edit anything about the event (except the recurring) from the GUI. Windows Calendar even knows it can't edit the more advanced recurrence rule: