Winter solstice

The Winter Solstice is the wintertime meeting between the gods of Olympus.

History
During The Titan's Curse, the Winter Solstice was to be the meeting place where they would discuss the treat of their father Kronos, and how best to deal with him, and then they would vote on whether they would take action or not.

This was important to the campers of Camp Half-Blood. Artemis, the goddess of the Hunt, was captured by Atlas, and she was intramental in the voting process, as she understood what a threat Kronos was, and how quickly he was mobolizing his army.

Percy, Thalia, and Grover managed to rescue Artemis, and she brought them to Mount Olympus, just in time for the voting. The gods voted in favor of action, and not destroying Percy and Thalia. At this meeting, Thalia joined Artemis' Hunters, as she was afraid of the prophecy, and knew that she would be weak where Percy wasn't.