The TrickClick on the first day of the series in Google Calendar and type 'My Event, fifth Sunday of every month, 12:30-3:00' in the "What:" field:
Modify details as necessary.
The StoryI use Google Calendar as my primary calendar. Having an Android phone, this is quite convenient, as I can not only pull up my calendar at any time, but am also able to set reminders which deliver notifications to my phone.
Just today, I ran into a problem: how do I get an event to repeat on every 5th Sunday of the month? Through the Google Calendar interface, the only options for repeating seemed to be on the 1st, 2nd, 3rd, 4th, and last Sunday of the month.