Well, it is all started that I wanted to add a recurring event for the world famous La Tomatina of Spain, where people throw tomatoes to each other in a sunny day of Valencia. The event is held on the last Wednesday of August, during the week of festivities of Buñol. Unfortunately, there is no way to add a recurring action like this on the website of Google.
However, the Google Calendar can read and understand successfully the iCalendar format, not to be confused with Apple iCal, so you can make use of it to achieve what you want. All you need to do is to create an iCalendar event with the below recurrence rule. The term -1WE, means last Wednesday of the month, and the rest is self-explanatory. You can find more information in the related RFC.
You can paste the below text into a new text file and save it as latomatina.ics. Then, you can go Settings page of Google Calendars, click on Calendars, and then click on import. You will need to select the latomatina.ics, and the calendar you want this event to appear. Now, it is done. You can edit this event on your calendar. However, you should not edit the recurrence part on the web interface of Google Calendar.
BEGIN:VCALENDAR BEGIN:VEVENT DTSTART;VALUE=DATE:20120302 DTEND;VALUE=DATE:20120302 RRULE:FREQ=YEARLY;BYDAY=-1WE;BYMONTH=8 DESCRIPTION: La Tomatina Festival LOCATION:Buñol\, Valencia\, Spain SEQUENCE:0 STATUS:CONFIRMED SUMMARY:Spanish Tomato Festival TRANSP:TRANSPARENT END:VEVENT END:VCALENDAR