''[[http://www.fanfiction.net/s/6783865/1/The-Day-Everything-Changed The Day Everything Changed]]'' is the first part of a three part ''Manga/LuckyStar'' Konata and Kagami story arc fanfic, and was written by [[http://www.fanfiction.net/u/2689121/Mr-FanficGuy Mr. Fanfic Guy]]. It was originally published on February 27, 2011 as a single huge chapter. Between April 14, 2012 and April 16, 2012, the author published a [[http://www.fanfiction.net/s/8026135/1/The-Day-Everything-Changed-Revised-Edition "Revised Edition"]], splitting the story into chapters like all his other fanfics, and adding some extra material.

The story starts out on [[TwentyMinutesIntoThePast Thursday June 30, 2005]], the day before classes at Ryouou High School let out for summer break. Konata has had a major crush on Kagami for the last three years, but she has always been too scared to tell her best friend her true feelings. At the same time, another student by the name of Sakura Takahashi, holds a grudge against Kagami for some reason, and she feels that Kagami somehow wronged her. And the next day, Friday July 1, she makes her move.

That morning, Sakura injects a mixture of Anadrol 50 and Dieselbolan v2.0 (two of the most potent steroids in the world) into herself and later advances on Kagami with a metal pipe in one hand and a pocket knife in the other, and mercilessly and senselessly attacks the poor girl. What makes it worse is that this scene plays out exactly like a nightmare Konata had the previous night. An absolutely ''furious'' Konata then fights back against Sakura, even taking several brutal hits herself. It's an intense fight, but eventually Konata, along with Tsukasa and Miyuki, manage to win the battle, and Sakura is arrested shortly afterward.

However, despite Konata's efforts, Kagami is still injured badly enough that she has to be taken to the hospital. Once they are there, Konata finally admits to Kagami that she's in love with her. At first, Kagami doesn't know how to respond, but that night, she accepts and tells Konata that she loves her as well.

The author has created two LighterAndSofter sequels: ''Fanfic/HolidayVacation'', which details the main four characters' third year beyond the final episode of the original series through the New Year, as well as the blossoming relationships between Tsukasa and Miyuki, and Yutaka and Minami, and ''Fanfic/LuckyStarAfterStory'', detailing the casts' post-college lives, the marriages of the three couples, the backstories of several original characters, and the birth of Konata and Kagami's daughter. Also by the author is the Music/{{Vocaloid}} fanfiction ''Fanfic/FromConcertToChaos'' (which is darker than ''The Day Everything Changed''), and the ''Manga/{{Nichijou}}'' fanfiction ''Not a Normal Day'', in which the Professor gets appendicitis.
!!This fanfic provides examples of:

* BadassAdorable / CuteBruiser / TookALevelInBadass: Konata definitely qualifies, as she shows no hesitation in fighting back against Kagami's attacker.
* BerserkButton: DO NOT put Kagami in harm's way, ''NEVER'' laugh at her pain, and '''DON'T EVER''' make her cry. Konata will be ''absolutely fucking'' '''PISSED''' if you do any of those things. Sakura did all of them.
* BigBad: Sakura Takahashi.
* [[BigDamnHeroes Big Damn Heroine]]: Konata when she fights back against Sakura.
* TheBully: Sakura is a particularly nasty and realistic example.
* CueTheRain: Right around the time Sakura moves in on Kagami, it starts to rain.
* CurbStompBattle:
** Poor Kagami didn't stand even the slightest chance against Sakura.
** Also to a smaller extent, Konata versus Sakura.
* DarkFic: Not ''nearly'' to the extent of something such as ''Webcomic/{{Lonely Kagamin}}'' or ''Fanfic/FallingStars'', but it still counts. It also helps that the weather the day of the attack [[ShownTheirWork really was overcast and rainy]].
* {{Determinator}}: Konata when she is fighting against Sakura.
* DisproportionateRetribution: Sakura had held a grudge towards Kagami for god knows how long. The moment she can no longer take it is when Kagami scored higher than her on the midterm. What does Sakura do? Beat the living daylights out of Kagami, and injure the poor defenseless girl badly enough that she has to be taken to the hospital.
* EvenEvilHasStandards: Sakura, while plotting, states that although she'll make Kagami suffer greatly, she will stop short of murdering her. Though given how she was clearly undergoing SanitySlippage during the attack, she probably would have killed her had Konata not intervened.
* GrayRainOfDepression: The clouds open up and rain starts to pour right around the time Sakura starts her assault on Kagami.
* JerkAss: Sakura, to the extreme.
* KickTheDog: Sakura does this brutally to Kagami during Chapter 4. The most severe example is when after Kagami starts crying and begs her to stop (while Sakura is twisting her already broken arm), Sakura responds by slashing her with her knife again.
* LaserGuidedKarma: Sakura is immediately arrested after the attack, and she is soon sentenced to 20 years in prison.
** Not only that, but a mob of angry students tried to get at her as she was being led out of the school by the police. They might have succeeded at lynching her, if it hadn't been for the faculty joining forces to keep them from attacking.
* LaughingMad: During Chapter 4, after hearing Kagami beg her to stop, Sakura starts laughing in a "borderline maniacal" manner, [[KickTheDog and then she slashes Kagami with her knife yet again]], [[NothingCanSaveUsNow and to top it off she boasts that no one can save her now]].
* NoHoldsBarredBeatdown: Sakura gives a brutal one to Kagami in the climax of the fanfic.
* OhCrap: At least 3 moments.
** Kagami, when she realizes that Sakura is about to attack her.
** Konata, when she figures out that her nightmare has become a reality.
** And finally, Sakura, when even after she strikes Konata several times with the pipe, she still refuses to give up fighting.
* [[OriginalCharacter Original Characters]]: Ayaka Suzuki and Nanami Tanaka are introduced in this fanfic right after the attack scene. They would later take up bigger roles in the next two fanfics.
* PropheciesAreAlwaysRight: The climatic attack scene plays out very similarly to a nightmare Konata had experienced the night before.
* RedEyesTakeWarning: When Konata starts fighting back against Sakura, she has what the author describes as a "bright red glow of rage" in her eyes.
* TheResenter: Sakura serves as this towards Kagami.
* SecretRelationship: Konata and Kagami agree to keep their relationship under wraps for a bit, only for it to become common knowledge in the sequel. Beforehand, though, the Les Yay between the two in the actual series can be chalked to them actually them struggling to keep it secret.
* TooDumbToLive: During her trial, Sakura makes no attempt to rationalize what she did, insisting that Kagami somehow had it coming and openly bragging that she "taught that bitch a lesson". She ends up getting 20 years in prison.