There are two red-letter days this month in the Common Worship calendar: Luke on 18 October (about whom I’ve written before), and Simon and Jude on 28 Oct. All three are apostles, which is why their commemorations are marked as principal feasts. But in fact we know very little about Simon and Jude from the gospels and, although there are some legends associated with them, this is sparse compared with some other apostolic saints. As a result, they don’t figure prominently in Christian art either. It’s odd that one feels the need to begin by making clear who they are not. Simon is not Simon Peter; and Jude is not Judas Iscariot. The gospels are careful about this too.
In the lists of apostles in Matthew 10, Mark 3, and Luke 6, the two Simons are distinguished by the name of Peter being attached to the first, and the second being identified as Simon the Zealot. In Luke this identifying term is ζηλωτης (zelotes) from the Greek ζηλος (zelos), meaning ‘hotness’, the corresponding verb meaning ‘to seethe’, or ‘to boil’. This is also the way this Simon is identified in the list of apostles in Acts 1 — unsurprisingly, given that Acts and Luke are considered to have the same author. In Matthew and Mark, on the other hand, the identifying term is (in transliteration) kananaoios or kananites, depending on the manuscript. This was misinterpreted early on (including by Jerome, no less!) as indicating that Simon was from the town of Cana, or from the region of Canaan. This is what we have in the King James Bible: Simon the Canaanite. But kananaoios or kananites is actually an attempt to record in Greek the Hebrew word for ‘zealous’, qanai when written in our alphabet. So modern English bibles take account of this and distinguish Simon as ‘zealot’ in all three synoptic gospels and in Acts; they are all saying the same thing about him, but using different words, one pure Greek and the other representing Hebrew. ‘Zealots’ was a name given to a Jewish party of revolt against the Romans round about 70 AD, but this is unlikely to be relevant for Simon the Apostle. For him ‘zealot’ is probably a personal description, ‘the zealous’, perhaps originally in keeping the Law of Moses, and then in following Christ.
Jude is even more elusive. In the list of apostles in Luke 6 he is referred to as ‘Judas of James’, most naturally taken to mean — and usually translated as — son of James, though we don’t know who that James was. In John 14 he is ‘Judas, not Iscariot’, which points to why we generally modify his name from Judas to Jude: it makes the distinction clearer. But the lists of apostles in Matthew and Mark don’t include a Jude/Judas in addition to Judas Iscariot. Instead, they include a Thaddeus or Lebbaeus (depending on the manuscript). The general supposition is that this is an alternative name for Jude. In some church traditions Thaddeus is the name standardly used, as we see from dedications and shrines, neatly eliminating any risk of confusion.
Beyond this there is not much that can be said, except that there is no firm evidence for Jude being the author of the Epistle bearing his name. According to legend Simon and Jude were martyred together in the town we know as Beirut (then in the Roman province of Syria), and so that is why we commemorate them on the same day.