The number thirteen represents Judas, the thirteenth to arrive at the Last Supper. Friday by itself is unlucky because it was the day of Christ’s Crucifixion. Years ago, the British set out to disprove these superstitions. They named a new vessel HMS Friday, laid her keel on a Friday, and then sent her to sea on a Friday that fell on the thirteenth. The plan backfired: neither ship nor crew was ever heard from again. Then, of course, there’s Apollo 13.