Epan is a compound conjunction combining epi (upon) + an (a conditional/temporal particle). It means "when" or "as soon as," marking the moment when a condition is fulfilled. It appears 3 times in the NT, always in contexts that carry a sense of urgency or immediacy.
The small connective words of Scripture carry theological weight. Epan marks the pivotal "when" moments — the hinge points on which action turns. In Matthew 2:8, Herod cynically uses it ("when you find him, report to me") while the Magi unknowingly serve God's purposes. In Luke 11:22 and 11:34, Jesus uses it in His teaching on spiritual power and the eye as the lamp of the body. The word reminds us that God's timeline is precise — there is always a "when" on His calendar, and it arrives exactly on time.