Festivalis (The Winter Solstice)

Festivalis is a five day, world-wide celebration of the change of the winter season and the coming of the new year. It occurs directly after the last day of Dantu (the 30th) and is followed by the 1st of Jywer. It is celebrated similarly to Earth's Christmas or Hanukkah holidays, with special festivities on the day of the Winter Solstice, the shortest day of the year.
The Winter Solstice can happen on the second, third, fourth, or fifth day of the festival week, though second or fifth day solstices are rare. The last fifth day solstice was in 560 A.U. and will not happen again until 960 A.U.
The official name of this five day week is Festivalis, which is used like a month name for any children born during this time. E.g. Clarity McCoghlan was born on the 5th day of Festivalis, 654 A.U.
Every fourth year, an extra day is added to the week of Festivalis to keep the 365 day calendar from becoming misaligned with Issolum's rotation - a practice that was instituted a little over three centuries ago.