Beren of the House of Bëor weds Lúthien of Doriath
Event Type: Genealogical
Age: 1st Age
And it seemed to Thingol that this Man was unlike all other mortal Men, and among the great in Arda, and the love of Lúthien a thing new and strange; and he perceived that their doom might not be withstood by any power of the world. Therefore at the last he yielded his will, and Beren took the hand of Lúthien before the throne of her father.
The Silmarillion, Quenta Silmarillion, Ch 19, Of Beren and Lúthien
The date comes from War of the Jewels, The Grey Annals, though in that version a very similar paragraph ends "and thus they were betrothed." Since some timelines I have seen give the date of the marriage as 469, it is possible that they are using the date that Beren and Lúthien went to Tol Galen and "dwelt together for a time as living man and woman."