Mereth Aderthad was the great celebratory feast held at the order of High King Fingolfin in the twentieth year after the first rising of the Sun. The feast was held by the Pools of Ivrin, under the Mountains of Shadow to the north of Beleriand. Its purpose was to unite all of the divided Elves in one cause, and ambassadors came from all the peoples and realms of Beleriand. As well as Elves from the separate branches of the Noldor, native Grey-elves were also present, including some from Doriath and the Falas. Ambassadors even came from the Green-elves, the people of distant Ossiriand in the east.
The Feast was hailed as a great success, and the Elves made common cause there against their great enemy to the north. For a few short years after the Mereth Aderthad, Beleriand enjoyed a time of peace and happiness before it was plunged into the Wars that would eventually destroy it.