The Midnight Cry is that exceeding bright light that God gave to the Advent people for 3 months between July 1844 to October 22, 1844. This cry is found in Matthew 25:6: "And at midnight there was a cry made, Behold, the bridegroom cometh; go ye out to meet him."
History:
William Miller & associates predicted that Christ would return to this earth between March 21, 1843, to March 21, 1844.
Since Christ did not return on March 21, 1844, the Advent people received their first disappointment in the Spring of 1844.
At the beginning of the Spring of 1844, the Advent people entered a prophetic period known as the "tarrying time". As it is written: "For the vision is yet for an appointed time, but at the end it shall speak, and not lie: though it tarry, wait for it; because it will surely come, it will not tarry." Habakkuk 2:3. During this period, many people left the movement to join the ranks of the enemies, casting reproach towards God's people that are still waiting for their Lord.
For the people of God, this "tarrying time" was like a Night to their experience, in that they were searching and re-examining their positions to find out what was their mistake and why Jesus had not returned.
3 months into the tarrying time, God sent an exceeding bright light to His people, and as a result, the Adventists discovered the mistake in their former reckoning of the prophetic periods, and settled upon the correct position
1 day prophetically is 1 year, 1 night prophetically is 6 months, and midnight prophetically is 3 months. And 3 months into the night (tarrying time), the cry was raised by Samuel Snow in the Exeter Camp Meeting: "Behold, the bridegroom cometh; go ye out to meet him on October 22, 1844".
While the midnight cry is NO LONGER sounding, the light and understanding God gave His people in how to understand His word, in this case: "midnight", and His calendar is still relevant today.
Ellen White's account of the midnight cry: Story of Redemption Page 369,Paragraph 2-3 IN THE SUMMER of 1844 Adventists discovered the mistake in their former reckoning of the prophetic periods, and settled upon the correct position. The 2300 days of Daniel 8:14, which all believed to extend to the second coming of Christ, had been thought to end in the SPRING OF 1844; but it was now seen that this period extended to the AUTUMN OF THE SAME YEAR, and the minds of Adventists were fixed upon this point as the time for the Lord's appearing. The proclamation of this time message was another step in the fulfillment of the parable of the marriage, whose application to the experience of Adventists had already been clearly seen. As in the parable the cry was raised at MIDNIGHT announcing the approach of the bridegroom, so in the fulfillment, MIDWAY between the SPRING OF 1844, when it was first supposed that the 2300 days would close, and THE AUTUMN OF 1844, at which time it was afterward found that they were really to close, such a cry was raised, in the very words of Scripture: “Behold, the Bridegroom cometh; go ye out to meet Him.” Early Writings 42.3: ...therefore Christians who fell asleep before the door was opened into the most holy, when the midnight cry WAS FINISHED, at the SEVENTH MONTH, 1844, and who had not kept the truth Sabbath, now rest in hope.