Noon, and the light went out
Three of the four Gospels record the same thing. Jesus was crucified in the morning. Then, in the middle of the day:
"And it was almost the sixth hour: and there was darkness over all the earth until the ninth hour. And the sun was darkened; and the veil of the temple was rent in the midst."
Luke 23:44 to 45
The Jewish day counted from sunrise. The sixth hour is noon. The ninth hour is three in the afternoon. For three hours, at the brightest time of day, the land went dark.
Matthew adds that the earth shook and the rocks split (Matthew 27:51).
It wasn't an eclipse
Some have tried to call it an eclipse. It can't have been, for two reasons.
Passover falls at full moon. An eclipse of the sun only happens at new moon, when the moon passes between the earth and the sun. At full moon, the moon is on the other side of the earth.
And an eclipse of the sun lasts a few minutes, never three hours.
So whatever darkened the sky that afternoon, it wasn't the moon.
Thallus, around AD 52
Thallus was a historian who wrote a history of the world in Greek, around AD 52. His book is lost, but a Christian writer named Julius Africanus quoted it around AD 220.
Africanus described the darkness at the crucifixion, then added:
"This darkness Thallus in the third book of his History, calls, as appears to me without reason, an eclipse of the sun."
Thallus wasn't a Christian. He was trying to explain the darkness away. But to explain something away, you have to admit it happened.
Around twenty years after the cross, a pagan historian was already dealing with the darkness as something people were talking about.
Phlegon saw day turn to night
Phlegon was a Greek writer in the 2nd century AD, a freed slave of the Emperor Hadrian. He wrote a history arranged by Olympic Games, and in it he recorded:
"In the 4th year of the 202nd Olympiad, there was a great eclipse of the Sun, greater than had ever been known before, for at the sixth hour the day was changed into night, and the stars were seen in the heavens. An earthquake occurred in Bithynia and overthrew a great part of the city of Nicæa."
The 4th year of the 202nd Olympiad is around AD 32 to 33, the years of the crucifixion. The sixth hour, noon. Day turned to night. And an earthquake.
Julius Africanus summed up what Phlegon reported: "in the time of Tiberius Caesar, at full moon, there was a full eclipse of the sun from the sixth hour to the ninth."
Full moon. Sixth hour to the ninth. Exactly what the Gospels say.
Amos saw it first
Around 760 BC, the prophet Amos was given a vision of a day of judgement:
"And it shall come to pass in that day, saith the Lord God, that the sun shall go down at mid-day, and I will make the earth dark in the day of light... and I will make it as the mourning of an only son."
Amos 8:9 to 10 · around 760 BC
The sun going down at midday. The earth dark in daylight. Mourning for an only son.
What it did to the soldiers
The Roman soldiers at the cross had seen plenty of men crucified. This time was different:
"Now the centurion, and they that were with him, watching Jesus, having seen the earthquake and the things that were done, were greatly afraid, saying: Indeed this was the Son of God."
Matthew 27:54
Why this is a big deal
The darkness wasn't a quiet detail only Christians remembered. A pagan historian tried to explain it away within about twenty years. Another recorded day turned to night, with stars in the sky and an earthquake, in the very year of the crucifixion.
It happened at full moon, when no eclipse was possible, at noon, when the sun should have been at its brightest. And eight centuries earlier, Amos had seen the sun go down at midday, and the mourning for an only son.