Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
The loop is actually purgatory.
- And instead of living the same day over and over again, she's actually trapped in her dying mind or in hell, forced to relive the day of her death until she can put to ease the mystery of her murder and finally have her soul put to rest. That would explain why the killer is seemingly omnipresent, always managing to find some way to kill her no matter how prepared she may be. In reality, a murder like this must have been premeditated and require intense planning. It would at the very least be really hard to simply turn around and rework your entire murder scheme in the span of one night. But that's because this isn't the actual killer. The actual killer succeeded at the beginning of the movie, and the one we'll see throughout the rest of the movie is merely a representation of the real killer.
- Jossed, once the murder's solved Tree does get a tomorrow.
Alternately, the killer is also looping.
- If Tree simply left to some random location, the killer should just turn up in her room like he did the first time only to find she isn't there. Instead the killer's looping as well and believes permanently killing Tree will end the loop. Because honestly, just how many times could you kill a person before you stop caring?
- Jossed, the killer just knows where Tree is no matter where she goes because she's her roommate.
The constant loop is the universe teaching Tree An Aesop.
Basically Tree is not a very nice person in general so it's more or less the universe giving her a Twilight Zone lesson about becoming a better person and to start changing her ways.- Kinda right - the loop where Tree Took a Level in Kindness is the one that goes right... if only because it makes her notice her roommate is trying to kill her with a poisoned cupcake. And in spite of the murder that ensues, Tree is still a better person afterwards.
- Jossed by the sequel which shows the loop to be the unexpected result of a science experiment.
Tree is a witch but doesn't know it.
Tree has an unconscious ability to use magic, which is why her looping is never explained. She's the one casting the spell that does it.- Jossed in the sequel, it's sci-fi instead of magic - namely, a machine exploiting space-time that opened the loop where Tree got caught.
Tree is looping because it is her birthday
The reason Tree gets to keep reliving the day she died is because she was killed on her birthday. The confluence of life energies connected with the day of her birth conflict with her sudden death and allow her to come back and try again, albeit with damage done to her.
- It's the universe's birthday present to her.
- Jossed by the sequel which shows the loop to be the unexpected result of a science experiment.
Tree is looping because of her mom.
Tree and her mom shared the same birthday. It makes sense that her mom would be looking out for her, especially on this day.
- She's literally the Birthday Girl.
- Jossed by the sequel which shows the loop to be the unexpected result of a science experiment.
The Day continues to loop because Tree has a version of Bites Za Dusto
Every time Tree dies, Bites The Dust goes into effect, resetting the day until Tree successfully finds out her killer's identity. The only thing is that Tree has no idea about Killer Queen's Existence.- Jossed by the sequel which shows the loop to be the unexpected result of a science experiment.
Tim isn't gay.
He just figured that since girls get to experiment then why shouldn't guys?- He could be just bisexual. That's why he can still be interested in dating Tree and watching gay porn.
Danielle will be the killer in the sequel
In the first movie, it never made sense why Tree stopped considering Danielle as a potential killer. They're just hit by a truck, and Tree crosses her name off. Plus, Danielle is confirmed to appear in the second film, and the fact that there's likely going to be a new killer in the movie, leads me to think Danielle is the logical choice.- Jossed. Danielle is indeed in the sequel but simply remains a bitchy airhead rather than an actual killer. In fact she ends up helping Tree at one key point.
Danielle will be the killer in the third
She's already been marked to be put in a time loop. She'll use her own experience in a new time loop to become a killer for fun. Only being a stopped by an alt!Tree.