From Tolkien Gateway

Evernight was a dark, raging part of the ocean, through which Eärendil passed on his epic journey to Valinor. The name only appears in Bilbo Baggins's 'Song of Eärendil', making it hard to identify with certainty. From its context in the poem, though, it seems to be a reference to the Shadowy Seas that formed part of the defences of Aman.