While I am sure its not a complete list (the only people who would have that would be Apple), here is a list I found on many of the phrases Siri understands:
- Set up a meeting at 9
- Set up a meeting with Michael at 9
- Meet with Lisa at noon
- Set up a meeting about hiring tomorrow at 9am
- New appointment with Susan Park Friday at 3
- Schedule a planning meeting at 8:30 today in the boardroom
- Move my 3pm meeting to 4:30
- Reschedule my appointment with Dr. Manning to next Monday at 9am
- Add Lisa to my meeting with Jason
- Cancel the budget review meeting
Asking about events
- What does the rest of my day look like?
- What's on my calendar for Friday?
- When is my next appointment?
- When am I meeting with Michael?
- Where is my next meeting?
More available at http://www.iphonehacks.com/2011/10/iphone-4s-features-exhaustive-list-of-phrases-siri-can-understand.html
I personally have found that many of these features are available through the UI, but so complex or have a confusing workflow that most people wouldn't deal with it (for example, think of how many taps it would take to set a meeting at a time somewhere and invite someone?). For example, in the linked question about every three days, I wonder if it set a repeat that way (which is not available in the UI), or created new independent events.
After seeing your updated question, related directly to recurring monthly events on a certain day-of-week, I could not reliably get this to work. Using a phrase like 'schedule _ for the first Thursday of every month' did prompt for a time, but then tried to create the even for every day. This may work on a month to month basis, as in 'schedule __ for the second Tuesday of April'. I did another test just now, and it actually created a repeating event, but nothing more than a repeating one on the 30th of every month (it said it was creating a new event starting today). This was using the same phrase, and gave obviously different results.
I am not sure if this was due to a limit of Siri, or the fact that the recurrence options of Calendar on iOS is limited. This feature is easy to do on iCloud.com or in iCal itself.