Although some have named Arvandor the entire first Layer of Arborea, just as others have named it Olympus, it is more specifically the Realm of the Seldarine, the powers of the elves led by Corellon Larethian. Arvandor is mostly endless canopies of forest with huge clearings containing many idealized elven settlements. It has a day and night cycle matching that of the material plane.
Sources:
Planes of Chaos
Manual of the Planes (D&D 3.0)
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