In two places, Joel 2:31 and Acts 2:20, the King James Bible says that the sun will turn dark and the moon will turn to blood before the great day of the Lord comes. The Revelation 6:12 says the sun turns black and the moon becomes as blood.
Sometimes, when the moon is partially eclipsed by the earth, sunlight passing through the earth's atmosphere is refracted in such a way that the shorter red waves of the spectrum fall upon the moon. When this happens, the moon appears reddish-brown, as if covered in blood. The phenomenon is not uncommon and it will occur four times between April 15, 2014 and September 28, 2015.
Two Protestant ministers, John Hagee and Mark Blitz, have noted in independent works that every blood moon of this series will fall on an important Jewish Feast day, according to the traditional lunar calendar used by the Jews. Further, this has happened on three other occasions since 1492: after the expulsion of the Jews from Spain, after the founding of the modern state of Israel, and during the Six Day War which led to the reunification of Jerusalem under Israeli governance. Based upon the significance of the blood moon in prophecy and the close association of the phenomenon with Jewish history, Hagee and Blitz suggest that this occurence, too, is prophetically significant.