A week; seven nights. Fortnight, fourteen nights. These words are relics of the ancient Celtic custom of beginning the day at sunset, a custom observed by the ancient Greeks, Babylonians, Persians, Syrians, and Jews, and by the modern representatives of these people. In Gen. i. we always find the evening precedes the morning; as, “The evening and the morning were the first day,” etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894