Some modern biblical scholars place the crucifixion of Jesus by Roman troops on this day.
All four canonical gospels place his death during the time of Roman prefect Pontius Pilate, which limits the range of possible years from 26 to 36 CE.
Jesus death would have been before the conversion of Paul, 33-36 CE. A wide variety of specific dates have been calculated by different scholars, all for different and plausible reasons.
Crucifixion was popular with the Roman authorities as a means for dissuading crime and rebellion. It's a slow, agonizing form of execution that's designed to extract the maximum amount of suffering out of the victim in a very public and humiliating way. Death can require hours or days.
Crucifixion can be done in a number of ways, with the most popular being a simple upright pole or even a tree, or a pole combined with a crossbeam. The latter is what becomes associated with the crucifixion of Jesus, though there is no direct evidence that this particular form of crucifixion is what was used.