A feast held by Fingolfin for the Elves of Beleriand, translated as “Feast of Reuniting” (S/113), a combination of mereth “feast” and aderthad “reuniting” (SA/mereth).
Conceptual Development: This name first appeared as N. Mereth Aderthad “Feasts and Games of Reuniting” in Silmarillion drafts from the 1930s (SM/329). In The Etymologies, the noun N. mereth was rejected and replaced by bereth (Ety/MBER), but as pointed out by Christopher Tolkien, the noun was never changed in the narratives, so Tolkien likely reverted this change.
The word was changed to bereth in the Etymologies, but was never changed in the texts (cf. Mereth Aderthad and the compound Merethrond)