Jesus was crucified on the day of Preparation of Passover week (John 19:14,31). This was the day that the Passover lambs throughout Israel were slaughtered (Exodus 12:6). Jesus had to die on Passover, because He was the world's Passover Lamb (1 Corinthians 5:7)--something that had been predetermined, by God, from the creation of the world (Revelation 13:8).

Tradition dictates that Jesus was crucified on a Friday: Good Friday. I disagree; I believe that it was in the middle of the afternoon on Thursday, a few hours before sunset, at which time Friday began (since a Jewish 24-hour day begins at sunset). To read why I believe this, see Good Thursday.