From December 13, 11:00 a.m.–January 4, 11:00 a.m. PST the fabric of Sanctuary will be altered every 24 hours, revealing a new gameplay modifier via the in-game Message of the Day that will last for the next 24 hours. The 22 Nights of Terror holiday event can be enjoyed in all online game modes except for Classic Diablo II.
https://news.blizzard.com/en-gb/diablo2/23885524/22-nights-of-terror-holiday-event