Arc week is set to be the first in-game event for Season of the Drifter. As you might be able to guess from the name it’s going to be focused around the Arc element, which should mean we can expect plenty of change to our Arc Subclasses. The event will run from the 9th of April until the 16th of April, when The Revelry will begin.
Source: N4G PC Destiny 2 Arc Week Detailed