John 5:1 — After this there was a feast of the Jews; and Jesus went up to Jerusalem.
2 — Now there is at Jerusalem by the sheep market a pool, which is called in the Hebrew tongue Bethesda, having five porches.
3 — In these lay a great multitude of impotent folk, of blind, halt, withered, waiting for the moving of the water.
4 — For an angel went down at a certain season into the pool, and troubled the water: whosoever then first after the troubling of the water stepped in was made whole of whatsoever disease he had.
feast of the Jews (v. 1) — The feast is no longer called Jehovah's feast because it was just an outward religious ceremony.
The feast was probably Pentecost, one of three annual feasts for which Jewish males were required to travel to Jerusalem. John may not have named the feast because the events it prophecied weren't fulfilled during the period covered in his book.
sheep gate (v. 2 — Nehemiah 3:1) — The gate through which sacrificial animals were brought to the temple.
Bethesda (v. 2) = house of mercy
The last clause in verse 3 and all of verse 4 are not in the earliest manuscripts and were probably added later by someone other than John.
If verse 4 wasn't written by John (and it has always bothered me some) — There was probably a superstition that when the spring bubbled, the first one in would be healed. And since, as this passage shows, the healthier always got in first, the superstition probably wasn't often put to much of a test.
If an angel did move the water, perhaps it was to show the impotency of the Law to save — The crippled man was there 38 years! — and how Christ was far superior to the Law.