From Emperor of Japan: Meiji and His World, 1852–1912, by Donald Keene (Columbia U. Press, 2005), Kindle p. 159:
As a further step in cementing the ties between the emperor and his people, the emperor’s birthday was proclaimed a national holiday, the Feast of Tenchō [天長節]. Observance of the emperor’s birthday as a holiday had begun as far back as 775, but the custom had long since fallen into abeyance. Its revival at this time was thus another instance of the intention to restore ancient practices.
On October 23  it was announced that the nengō [年号] had been changed from the fourth year of Keiō to the first year of Meiji and that henceforth there would be only one nengō for an entire reign. The name Meiji was derived from a passage in the I Ching, the ancient Chinese book of divination: “The sage, facing south, listens to the world; facing the light, he governs.” The day before the new nengō was announced, the emperor himself had visited the sanctuary (naishidokoro [内侍所 ‘inner samurai place’]) where he drew lots to determine the new nengō from among several names submitted by scholars. Although he probably did not realize it at the time, the emperor had also chosen the name by which posterity would know him; earlier emperors were known by a place-name from the site of their residence or (as was true of Meiji’s father and grandfather) by a posthumously chosen title. The name Meiji [明治], interpreted as meaning “enlightened rule,” came to seem an accurate description of his reign. Names like those of his father and grandfather, auspicious though they were, would have been less appropriate to the era.