You can't "reserve a mooring" on Catalina Island. They are privately owned but, if unoccupied, rented out on a first-come, as-available basis. If the owner or a sublet of your mooring radios in they are on their way, you will be kicked off your (their) mooring -- possibly reassigned another (bumping someone else via some arcane seniority formula), possibly told to Go Fish. During the summer, expect the major moorings at Avalon and Isthmus to be full nearly every night. Sometimes you will be sent to Emerald Cove or one of the smaller mooring fields or anchorages; these may be lumpy.
I highly recommend Cast Off for Catalina
, a slightly cheesy but quite informative DVD that covers the protocols of boating to and at the Island, along with a widdershins circumnav, visiting each harbor or anchorage in turn.
It's our goal this summer, also. Hope to spend five days there, maybe even do the loop. The other (northern) Channel Islands are rather more severe: no services, strictly limited access, rugged coasts with few harbors, and much rougher weather and currents. We decided Santa Catalina will provide sufficient challenge for our first Big Water experience.