According to director Christopher Landon, Tree's punchline, "Who takes their first date to Subway? It's not like you have a foot-long," was improvised by Jessica Rothe.

The Bayfield University Baby killer costume was designed by Tony Gardner, who also designed the Ghostface mask, the horror icon of the Scream franchise and which is a reworked version of the Father's Death Halloween costume.

Director Christopher Landon tested wearing the option of a baby mask for the killer in the office and scared a worker, confirming his choice.

The scene where Tree walks through the campus quad naked had to be done quickly; given that it was being filmed on an active college campus, this presented the risk of students witnessing the scene being filmed and/or taking photos. The crew took extreme precautions to clear away any potential onlookers. In the end, they managed to do just 2-3 takes.

The theatrical trailer utilizes the song "In Da Club" by 50 Cent as Tree's birthday ringtone. However, the final film does not feature this as the crew and studio could not acquire the rights. As a result, an original ringtone composition was concocted.

When asked why a baby mask: Christopher Landon says he needed a combination of something that would pass for a mascot on a college campus, that was both scary and funny at the same time, plus he was expecting a son at that time, so he had "baby on the brain."

The original script written by Scott Lobdell included material intending to make the film R-rated. Some scenes involved grislier death depictions that were entirely altered by the film's director Christopher Landon.

The original mascot and mask designed by Tony Gardner for the killer was a pig, a motif already done in the Saw movies.

Jessica Rothe described her nude scene as "liberating" because an all-female film crew shot the scene.

The scenes where Tree leaves Carter's dorm room and walks through the campus quad, with the same specific events happening each time (i.e. The couple getting soaked by the sprinklers, the car alarm going off, etc.) took two days to shoot. At one point, the sprinklers malfunctioned and couldn't be turned off, costing the crew at least an hour of filming until they were finally able to fix it.

Even though they only filmed and show her face and bare back in the movie, Jessica Rothe said in a 2017 interview that she decided to go totally nude for the scene where Tree walks around campus naked in order to feel the same rush as her character. She called it a very "liberating" moment. "It was incredibly freeing. I think everyone fantasizes about what that would be like if you kind of just got to live life without repercussions and do whatever you want, and what that bucket list would be. Not only did I get to play someone doing it, but I got to do a few of those moments, like the naked walk of glory--not walk of shame, walk of glory. It was one of the most empowering experiences I've ever had in my life. And a huge part of that was I trusted [the director] so much, and to portray that in a way that I felt comfortable. The crew was incredible." She explained that when they shot that scene, all the women of the crew would surround her holding up their jackets, shielding her from outside eyes and cell phones so footage of her stark naked couldn't be leaked onto the internet. The women would chant her name and she would do a little dance in the middle between takes to get pumped up. "Then as they yelled, 'Action,'" she added, "they would disperse, and I'd do my little walk, and they would all come and cover me up at the end. I just felt such love and female awesomeness in that moment. It's something where I could have a very long and successful career, and never get to do that again. So there were a lot of amazing moments filming this."

The script was written back in 2007. It was going to star Megan Fox and was to be produced by Michael Bay, which may explain the name of the school in the film, Bayfield University.

Tree's shirt says "Dumpstaphunk" on it. 'Dumpstaphunk' is an actual funk and jam band from New Orleans, Louisiana.

Originally titled "Half to Death."

Rachel Matthews (Danielle) is granddaughter of the late Michael Landon, famous for Little House on the Praire, and therefore she is niece to Christopher Landon (Director).

Joseph Tombs was the name of the high school gym teacher of writer Scott Lobdell.

During the film, Tree's full name is revealed to be Teresa.

The book the night nurse is reading is "A Seal at Heart" by Anne Elizabeth.

Tree's birthday is September 18th. You can see the date when she looks at the alarm clock and realizes she's late for class.

The creator of the freaky King Cake Baby character, which is used as one of the New Orleans Pelicans mascot has sued the creators of Happy Death Day for stealing his mask.

Danielle Bouseman's name is likely a reference to horror director Darren Bousman.

After Prom Night (2008) and The Final Girls (2015), this is the third slasher film ever to receive a PG-13 rating.

An original ending featured Tree Gelbman succumbing to her accumulated injuries after she kills Lori, her real killer. She is being treated for said injuries by her professor's wife before being murdered again by her, as revenge for sleeping with her husband, thus entering another time loop. The test audiences reacted furiously to this ending, feeling betrayed that the protagonist, after becoming a better person, could not break free. This ending was conceived as a way of leading the viewer to believe it was never going to end. It was scrapped and reshot to the current theatrical ending as a result. Ironically, the sequel ignores the new ending because it opens with the scene where Ryan catches Tree and Carter kissing on the bed and Carter tells him to get out.

An explanation for why Tree got stuck in the time loop in the first place is never given. But director Christopher Landon did include it in the sequel.

In Tree's supposed last loop where she fixes everything including her relationship with her roommate, Lori does not give her the cupcake. She may have been disoriented from Tree's apology because she later releases John Tombs from his restraints, who then goes on to attack Tree as before. This does not reveal that all Lori needed was an apology: she's still a psychopath.

When asked the cause of the time loop, Christopher Landon says there is a definite reason for it; and if the viewers look closely at the film, there are easter eggs pointing to the cause - and one is really big. But he did add a definitive cause in the sequel.

There are two clues in the film to the murderer's identity: first, in the first day, when Lori meets Tree at the hospital, knowing that she is going to meet Gregory, she says "I think something like this is bound to have some pretty serious consequences". Second, in the loop where Tree ends up dying in a car explosion, the murderer uses a birthday candle to light the fuel leaking from the police unit - similar to the candle on the cupcake which Lori made for Tree. This also proves that Tombs is not the killer because he doesn't know that Tree is celebrating her birthday, and therefore would have no reason to taunt her with a birthday candle.

When Tree and Carter are sitting across from each other and Tree eats the cupcake, the shot mirrors the final scene of Sixteen Candles (1984), with the boy and girl sitting cross-legged across from each other in front of a window with the candle(s) on the (cup)cake burning in between them.

Body Count: 19 - Tree dies 10 times on-screen; Nick, Danielle, Carter, Gregory, officer Santora, nurse Deena, Tombs' guard, Tombs and Lori - one time each. Although not shown on screen this should be 26, with Carter presumably dying a second time after sharing the poisoned cupcake in the penultimate loop and Tree technically dying 16 times, according to her dialogue, though not all are seen. In the end, the body count is 1 with Lori dying in the film permanently.

In Tree's supposed last loop where she fixes everything including her relationship with her roommate, Lori, it is hinted that she is actually the killer because that is the only time she chooses not to give Tree the poisoned cupcake after Tree has apologised. This also reveals that all Lori needed was an apology.

Carter points out the similarities between Tree's situation and the film Groundhog Day (1993). There are in fact a few scenes that directly parallel that film's events. Just like Phil, Tree proves that she has lived through this day by correctly pointing out several events about to take place around her. One of them is a frat brother passing out, whom she helps with a pillow. This parallels a scene in GD where a kid falls out of a tree, which Phil memorized and catches in one of his errands. Phil points out that a waiter in the diner is gay, and Tree eventually finds out that Tim is gay. Her repeated unwanted encounters with Tim are similar to Phil running into Ned Ryerson. Two of Tree's deaths involve getting hit by a bus and hanging herself in a bell-tower. Two of Phil's suicides involved walking in front of a bus and throwing himself from a bell tower. Lori's last name is also Spengler, which was the name of GD director Harold Ramis' character in Ghostbusters (1984), which also paired him with Bill Murray.

Taking inspiration from Uma Thurman's Kill Bill Vol. 1 (2003), the scene where The Bride comes to kill Vernita Green at the very beginning of the film mirrors the scene near the end of of this film when Tree decks out in a black leather jacket, blue jeans, and a knife to confront Joseph Tombs.