Next CECOS class is probably in January. They used to have them January, May, and September, but cut the May class in '06 due to funding.
The dates above only reflect the "basic class" dates. However, as a new Ensign, you should tack on a week (I think on the front end) for BOLTC and then 2 weeks after basic for your specialization class (ROICC, Public Works, Seabees), then another 2 weeks for CON 120. I was in last years September class, and we didn't get out until the Tuesday before thanksgiving (they modified 120 so it wasn't a full 2 weeks).
Don't bother preparing for CECOS...worry about OCS. CECOS will feel like a 3-month vacation once you finish OCS.