Complex iCal Event Repeat

Here's 1 way you'll can do that:

a) create the event you want on the 1st day you want, i.e., the 3rd Thursday in September, entering the description of the event

b) if the event is an all-day event, when u click on the Date showing for that event, you will then have the option to click "All-day" and the event will become sort of a "banner" above the day's "timed" events; if the event is going to have a specific duration, specify the start time and end time for that event

c) now here's the answer to your question...below the end time for the event, or the "all-day" check box, you should see "repeat:" with "none" as the default value; click on "none" and a drop-down menu should open. At the bottom of this you should see "Custom...", click on that. A little window will open: set the "Frequency:" to Monthly and the dialog box should expand giving you the ability to stipulate "every 1 month". Then below that you'll see 2 options: "Each" is the default value selected, which will have the event repeat every month on the specific date selected in the calendar; the other option, and this is the one you'll want to select, is "On the" and this is just below the calendar for the "Each" option. When you select the "On the" radio button, this will then activate the 2 drop-down menus below it. You'll want to select "third" and "Thursday" and click OK. Now reopen the event to continue editing it and below the "repeat:" setting you'll see "end repeat:". Simply change "Never" to the date you want the repeat to end and click OK. That should do it! Hope this helps!

What I just told you is based on me working on a desktop Mac, but it should be pretty much the same if you're doing this on ur iPhone...it should give you the same settings options as I've described.