'Days' in the Bible

The Lord sent a huge fish to swallow Jonah, and Jonah was in the stomach of the fish three days and three nights.

So Jonah went immediately to Nineveh, as the Lord had said. (Now Nineveh was an enormous city -- it required three days to walk through it!)

When Jonah began to enter the city one day's walk, he announced, "At the end of forty days, Nineveh will be overthrown!"