Analysis of information sources in references of the Wikipedia article "Endymion (mythology)" in English language version.
[...] as scholars have already perceived, Endymion is the sunken sun overtaken by the moon below the horizon, and his fifty daughters by her are the fifty lunar months of an Olympiad, or, more strictly speaking, of every alternate Olympiad.
Endymion is sometimes called the founder of the Olympic games, which links up with the legend that the moon goddess bore him fifty daughters, Pausanius 5.1.4. H. J. Rose (Oxf. Class. Dict. s.v.) sees this as a reference to the fifty months of an Olympiad.