Winter Solstice

The Winter Solstice is held during the wintertime meeting between the gods of Olympus and is the darkest day of the year. The solstice is a day when Ancient Evil magic is strongest. This is the only time when Hades can visit Olympus. It always occurs on December 21st.

The Lightning Thief
During the Winter Solstice, the Master Bolt was stolen from Zeus by Luke. Luke was on Mount Olympus with some campers from Camp Half Blood, so he had an alibi. He snuck into the throne room and stole the Master Bolt from Zeus and the Helm of Darkness from Hades. He found the theft incredibly easy, stating "The Olympians are so arrogant; they never dreamed someone would dare steal from them."

The Titan's Curse
The Winter Solstice is during the meeting in which the Olympians discuss the threat of their father Kronos, and how best to deal with him. They hold a vote on whether they should take action or not.

Artemis, the goddess of the Hunt, had been captured by Atlas, and she is essential in the voting process, as she understands what kind of a threat Kronos poses.

Percy, Thalia, and Grover manage to rescue Artemis, and she brings them to Mount Olympus, just in time for the voting. The gods vote in favor of action, and not destroying Percy and Thalia. At this meeting, Thalia joins Artemis' Hunters, partially in order to escape from the Great Prophecy.

Strangely, Hades isn't present, even though the Winter Solstice is the only time of the year he is allowed to visit Olympus.

The Lost Hero
The Winter Solstice is the deadline to save Hera from her cage since the gods would end in chaos fighting themselves without her "motherly figure."

Trivia

 * It is interesting to note that during the The Titan's Curse council, Hades was not present.