Some more details on the play, that took place on Dec. 23, 1972 at Three Rivers Stadium
The Pittsburgh receiver whom the ball was thrown to was Frenchy Fuqua. Jack Tatum was the Raiders defensive back who made the thunderous hit on Fuqua that sent the ball flying several yards away, where Harris caught it just before it hit the ground (arguably).
The rules back then were that a thrown ball could not touch two offensive players consecutively, so if the ball ricocheted directly off Fuqua to Harris, it would have been illegal. However, it was ruled the ball hit Tatum. Replays are inconclusive.
Tatum, in his 1979 autobiography "They Call Me Assassin", says "I couldn't honestly say if the ball hit me. I wasn't worried about the ball at the time. I just wanted to lay some wood on Frenchy and I did".
As for Fuqua, Tatum claims Frenchy told reporters "Damn, that sucker really hit me! I didn't know where the hell I was or even who I was, and you're asking me about the ball?"
Tatum concludes "There are advantages to the home field, and I'm sure the Steelers were enjoying theirs. If the game had been played in Oakland, that play would have gone down as an incompleted pass and we would have enjoyed our home-field advantage."
The name of the famed play has a story attached to it as well. According to a Dec. 26, 2000 story in the Pittsburgh Post Gazette, two fans named Michael Ord and Sharon Levosky were celebrating the win in a bar. An inebbriated Ord announced to the crowded bar "From here on, this day will forever be known as The Feast of the Immaculate Reception."
When they left the bar and ended up at Levosky's parents' home, they decided the name was too good to not share. Levosky called Pittsburgh sportscaster Myron Cope and told him of the name. So Cope, who was Jewish, used the catchy name that was derived from the Immaculate Conception on the air in his nightly sports report, and the name got picked up everywhere else soon after.