Cookie O'Clock is a special time during which the ThunderCats eat their favorite Thundersnaps Cookies. It is the Sword of Omens which determines when it is Cookie O'Clock.
During their first Cookie O'Clock on Third Earth, the ThunderCats were unable to eat any cookies as Lion-O was unable to open the jar of Thundersnaps. One thing led to another and the ThunderCats ended up on the Swamp of Boggy Ben where Panthro debuted the "Thundersnap Launcher" feature of the ThunderTank. So the ThunderCats were able to observe Cookie O'Clock after all.