Squidward is still asleep, only to be woken up and then annoyed by SpongeBob and Patrick, who came into his house to throw a surprise birthday party for him. When they leave, he shouts that it isn't his birthday and calls Patty Rechid from "Bikini Bottom Realty" to arrange for his house to be sold, so that he may move out of Bikini Bottom. He is warned that if his home is surrounded by bad neighbors, the house may not be sold. Squidward is convinced that SpongeBob and Patrick may ruin his chances of selling his house. Squidward thinks that whatever a good neighbor does, SpongeBob and Patrick do the opposite, which gives him an idea.
Squidward tells SpongeBob that it is "Opposite Day" in the morning, and that everyone must act opposite to how they usually act. SpongeBob believes him, and informs Patrick, who stops breathing, until SpongeBob exclaims, "not that opposite!" A montage of the two doing "opposite" things, such as talking backwards and acting as Gary's pets, is then shown.
Squidward's plot soon begins to backfire on him. SpongeBob "fixes" (damages) his house, and a shocked Squidward cleans the debris with a bulldozer. SpongeBob concludes that Squidward is acting like him, and he must therefore act like Squidward. While Squidward is still fixing SpongeBob' house, Patty arrives, and SpongeBob and Patrick both pretend to be like Squidward, giving her a tour of the house, while describing it negatively and doing the opposite of what she asks. When Squidward realizes what is going on, he rushes to his house and attempts to explain the situation to Patty, who, upon hearing that he is "Squidward" as well, finally sees through his game and accuses them of intentionally messing with her. Gary comes in with a pickle on his face, resembling Squidward's large nose. He meows with a Squidward-style voice, and she leaves. Squidward begs Patty to sells his house, only for her to yell at Squidward by saying "NEVER!" and she wouldn't sell the house if he was the "last Squidward on Earth" and storms off. SpongeBob and Patrick tells Squidward Happy Opposite Day and that they hate you. A furious Squidward shows SpongeBob and Patrick how much he "hates" them by attempting to run them over with a bulldozer, scaring SpongeBob and Patrick out of their wits as he is now on the warpath against them.
Despite it being Wednesday, SpongeBob and Squidward were not working at the Krusty Krab, but since SpongeBob actually thought it was opposite day he didn't go to work.
It is revealed in this episode that SpongeBob knows that Squidward dislikes him.
The scene where SpongeBob and Patrick change each other's colors may have inspired SpongeBob PinkPants, a pink version of the Spongebob doll sold in the UK around 2006.
The scene where Patrick danced around repeating "I'm Squidward" became popular on YouTube.
In the Russian dub, the line "Kcirtap yeh. Pu evig I. Edis etisoppo eht ot teg ot." was changed to "Кем мы были? Пузырями. Сами надулись и сами лопнули!" ("Who were we? Bubbles. We blew ourselves & popped ourselves!"). The line isn't spoken backwards.
Patty Rechid was voiced by Lori Alan, who also does the voice of Pearl in the series. However, the end credits mistake her to be voiced by Sirena Irwin.
When SpongeBob and Patrick are talking backwards they are saying: "Hey, Patrick", "I give up", and "To get to the opposite side". The captions says "(speaking gibberish)"
On the Drawn Together episode "Terms of Endearment", Wooldoor Sockbat has a similar joke based on this episode.
This episode, and "Nature Pants," aired exactly 2 years before the 9/11 attack.
In Indonesia, instead speaking backwards, SpongeBob and Patrick are saying the riddle of "Why did the chicken cross the road?"
This is the first and one of the few times that SpongeBob's belt buckle is shown.
If Squidward was acting as the opposite then he'd be acting like Spongebob and Spongebob would be miserable.
Also Patrick would be smart instead of acting miserable.
After Patty leaves, Squidward lays on the ground in shame. In the close up of his face, SpongeBob has his belt around his waist, but it was wrapped around his head.
When Patty says, "Are there anymore Squidwards I should know about?", the captions said, "Are there anymore Squidwards?".
In 1 scene, Squidward's house ear has window, then it's gone.
When SpongeBob puts the belt around his head, the belt buckle and the piece of belt that goes through it disappears.
When SpongeBob offers the Realtor to see the rest of the house captions said, "Would you like to see the rest?"
SpongeBob says to Patrick "That's right, you're my worst enemy!" but he should've said "That's wrong, you're my worst enemy!"
When Squidward is rebuilding SpongeBob's house, he was just started. When the view changes to SpongeBob for 1 second, the modeling is already finished. Or maybe Bikini Bottom time is faster then real time.
When Patrick first sings "I'm Squidward, I'm Squidward, Squidward, Squidward, Squidward" the flowers on his pants are almost black, but in the next shot they are dark purple - like in other episodes in this season.