The Day of Separation was an event described in the Klingon sacred texts which marked the discovery of the kuvah'magh. According to the texts, those that went in search of the kuvah'magh needed to "cast off the old ways" in order to follow him or her and truly build a new Klingon Empire.
The religious sect led by Kohlar in the 24th century encountered the USS Voyager in the Delta Quadrant. Kohlar believed that B'Elanna Torres' child was the kuvah'magh and declared it to be the Day of Separation. He therefore ordered his ship to self-destruct, thereby ridding them of the last remnants of the old corrupt Empire. (VOY: "Prophecy")