Got it, thanks! :)

Aloha Chris,
When you do this one, could you also create a permission for which usergroups can use the 'Reminders' section of the event creation screen. It's a dangerous thing for site members to use if they don't know what they're doing, since it causes email blasts.
One of my site members was trying to learn how to create a recurring event for the first time and inadvertently caused many reminder emails about the same event to be sent to all site members.
Thanks

Aloha,
I also noticed when masquerading as a site member and viewing this 'Reminders' section of the event creation screen, you see '(Secret usergroup #33)' and more in the usergroups list. I don't know if this is referring to usergroups that I had set to be hidden, but I think it would be confusing for site members to see this in the list

(Click to enlarge)

I've uploaded a screenshot to show how this now works. It's very simple... you just set the date, choose monthly recurrence, and then it autodetects some possibilities you might want. You don't have to work anything out, it does it for you.

Regarding reminder permission...

Added.

Regarding those hidden groups...

I think that was a bug already fixed in the latest code.

Tests...

The monthly recurrence settings can only be changed if monthly is set as the recurrence
The illustrated monthly recurrence settings adapt dynamically when the event start date is changed
An event with day_of_month_backwards recurrence shows correctly from month to month
An event with dow_of_month recurrence shows correctly from month to month
An event with dow_of_month recurrence shows correctly when viewed in full
An event with dow_of_month recurrence shows correctly for the side_calendar block's listing mode (do for a large number of days to check it's recurrence shows correctly)
An event with dow_of_month_backwards recurrence shows correctly from month to month
Going back to edit an event with dow_of_month recurrence shows the correct settings
Recurrence is probably encoded in iCalendar export for a 2nd Tuesday of month event - iCal can import it
An iCalendar export produced from iCal with a 2nd Tuesday of month event imports correctly
Users without "Set calendar reminders on behalf of other users" permission cannot do so

Hi Rishi,

I've uploaded the patch, but I expect to be updating you with all your recent sponsored stuff soon. I'm doing some juggling, because I want you to benefit from it all in one go, and because a reimplementation of your special member directory with the new Composr filtering functionality will be worthwhile for you.