Intertwention Day is a New Day era holiday with roots in the Long Night. The festival is observed three times a year, whenever the 13th day of the month falls on a friday. According to the calendar adopted by the new Alliance, this happens in the 6th, 9th, and 12th months of the year. The different dates are named for the seasons, with Spring, Summer, and Winter days. Traditionally, Summer Intertwention Day is the most celebrated as it happens shortly after the Alliance schoolyear resumes.
The exact origin, and even meaning of the name, are lost to history. The celebrations started sometime in the last thousand years or so of the Long Night, and may in fact have been dozens of unrelated holidays. By the New Day era, the holiday had progressed to it's current form.
According to legend, when Arindell was first resettled, the many peoples brought together to populate the new city had several different ideas about when and how to celebrate Intertwention Day. All anyone could agree on was that it had to be held on a Friday on the thirteenth day of the month. Since this happened three times per year, Conri Jusenkyou declared all three dates to be valid.
By the end of the first century N.D., Intertwention Day had come to be associated with three things: costumes, candy, and getting scared. Some traditions held that each occurrence should be devoted to one of the three, but this is believed to be a modern invention. So little is actually understood about the holiday that its hard to draw any real conclusions.
Somewhere around the end of the first century, it became primarily associated with children, and specifically giving those children large quantities of candy, but it has always been a holiday for everyone.