The Year of Kahless was a designation used to label years in the Klingon calendar. It was presumably named after Kahless the Unforgettable.
While serving as executive officer on the Rotarran, Worf made an entry in his combat log at the 53rd day in the year of Kahless 999. (DS9: "Soldiers of the Empire")
This corresponded with the year 2373, somewhere between Stardate 50712.5 and stardate 50814.2.