What does Matthew 27:45 mean?

In Jewish Scriptures, darkness symbolizes God's judgment or great tragedy. In this case, it represents both. The prophet Amos delivered these words from God to Israel: "'And on that day,' declares the Lord God, 'I will make the sun go down at noon and darken the earth in broad daylight'" (Amos 8:9). In a similar way, God used a plague of oppressive, three-day darkness in Egypt to communicate His power and judgment against Israel's enemy (Exodus 10:21–23).

As the Son of God hangs on a Roman cross, dying for the sins of humanity, a sudden darkness falls over the land of Israel. The One being judged is Jesus Himself; He suffers the consequence for sins He never committed (Romans 5:18–21; 2 Corinthians 5:21). Commentors speculated about what might have caused the darkness, though conventional explanations don't seem to work. A natural eclipse of the sun is physically impossible near a full moon, as would have been the case close to Passover. No mention is made of clouds, fog, or other obvious weather issues. This is simply "darkness." Whatever mechanism was involved, God brought it about.

Roman time was aligned differently from that of Israel. Roman hours, as in most modern calendars, began at noon and midnight, while Jewish hours typically ran from the approximate hours of sunrise and sunset: 6 a.m. and p.m. According to Jewish reckoning, then, this darkness lasted from noon until 3 p.m. John's account only indicates that Jesus was brought to Pilate's place of official judgment around "the sixth hour." Many believe John was using the Roman time system, implying that event happened at 6 a.m.