Arnold Schwarzenegger's first film since Batman & Robin (1997). The reason for the gap was because of his heart surgery after playing Mr Freeze; the studios were anxious about whether or not they could insure him, and despite attempts to convince them he was in perfect health, he couldn't get any work until End of Days (1999). Even then, he was amazed when insurance people and executives from Universal came to the set just to watch him, to see if he was still up to the action scenes. They asked Schwarzenegger if he enjoyed this kind of punishment, but he said he was used to it. After the first week of shooting, the insurance guys backed off and not long after, the film offers started rolling in again.
Years later, Miriam Margolyes revealed on a talk show she did not enjoy her first and only working experience with Arnold, citing the odd reason that in their scene together "He farted in my face!" (as she so eloquently put it).
Udo Kier claimed in an interview that he was considered for the role of Satan in the film but the producers didn't want the hero and villain to both have German/Austrian accents, and he was eventually cast in a secondary role.
One of the priests is named Thomas Aquinas. Thomas Aquinas was one of the greatest philosophers, a 13th-century proponent of natural theology and the father of the Thomistic school of philosophy and theology.
There is a scene which depicts the dropping of a cigarette into a flammable liquid, which the camera follows as it burns, is similar to a scene in The Usual Suspects (1995), also featuring Gabriel Byrne and Kevin Pollak.
The second of four movies with the word "day" that had Arnold Schwarzenegger, in which three of them he had a starring role and the title has a biblical reference; The others were Terminator 2 (1991), The 6th Day (2000), and Around the World in 80 Days (2004).
The trivia items below may give away important plot points.
An alternate ending was filmed in which Jericho resurrects after impaling himself on the sword of the statue and then leaves the church with Christine. Test audiences preferred the ending where Jericho dies. Even though the "resurrection" ending was never released, it was used in a novelization of the movie.
In this film, Jericho Crane sets out to stop Satan from impregnating Christine York and giving birth to his child which must happen between 11PM and midnight on New Year's Eve. In Ghostbusters II (1989), which also takes place in NYC, the Ghostbusters set out to stop the evil sorcerer and tyrant Vigo from possessing the body of Dana Barrett's 8-month-old son Oscar and be reborn, which must happen before midnight on New Year's Eve.