Otter Day comes around every January 30. The idea behind this is to have a minor holiday to look forward to once the post-Christmas blues set in. In the setting of Otter island, Otter Day was created as a day to celebrate the joy of being an otter. In order to include non-lutrine friends, the idea was changed to the celebration of being a creature who can experience joy.

In the real world, Otter Day was created by Oren Otter has been observed since 2005.

The Otter Day observances begin the day before with the Otter day Eve endurance challenge. Otters have a full day to swim 23 miles out to sea to a small island where they spend a miserable night in the cold and the rain, followed by a long swim back. Those who make it all the way without needing to be rescued are hailed as heroes and given a white scarf to honor their triumph. Humans are invited to take part in a parallel event- an overland hike with approximately the same rules.

Before going to bed, cubs are encouraged to leave some seaweed out for the Waitoreke, a mysterious creature who visits in the night and leaves fish-shaped cookies in exchange for the seaweed. On Otter day, presents are sometimes exchanged. An unwritten rule says that they are to be handmade gifts, so as to be from the heart. Feasting is the high point of the holiday, as it is on every otter holiday, with the champions of the endurance challenge as the guests of honor.  Humans who observe the holiday are encouraged to celebrate by feasting on traditional lutrine foods, such as fish, eggs, poultry, fresh fruit, crustaceans, caviar, clams, oysters, urchin, octopus, seaweed and so on.

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