- Wild Scallop
- Squidward Tentacles
- SpongeBob SquarePants
- Patrick Star
- Seahorse (debut; on a poster)
- Patty Rechid (debut)
- Gary the Snail
- Clam (mentioned)
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 the next 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's 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 sell 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 then she storms off. SpongeBob and Patrick tells Squidward "Happy Opposite Day." 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.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ Grass Skirts Blowing - Jan Rap [title card]
‣ Hawaiian Link (b) - Richard Myhill [Squidward sleeping]
‣ Sunny Samoa - Jan Rap [Squidward on the phone with a realtor]
‣ On the Beach - Kapono Beamer [the next day...]
‣ Duncan Gray - Robert Alexander White [Squidward playing bagpipes]
‣ Hello Blues - Sage Guyton, Jeremy Wakefield [Squidward tells SpongeBob about Opposite Day]
‣ Hawaiian Holiday - Kapono Beamer [SpongeBob gets into bed]
‣ Chief Taravana - Kapono Beamer [Patrick at the door]
‣ Nostalgic Hawaii - Jan Rap [doing everything opposite]
‣ Good Bye Samba (a) - Ben Nabor [SpongeBob's house wrecked]
‣ Hawaiian Link (a) - Richard Myhill ["Isn't it beautiful?"]
‣ Brushes - Martyn David, Alan Bell, Roger Stephen Dexter [Squidward rebuilding SpongeBob's house]
‣ Timpani - Military (d) - Gary Kettel ["I don't get it."]
‣ Entry of the Gladiators - Dick Stephen Walter ["I must be the opposite of SpongeBob."]
‣ Drum Roll to Cymbal [#46] - Martyn David, Alan Bell, Roger Stephen Dexter [drum roll]
‣ Vibe Link (b) - Richard Myhill [SpongeBob snaps fingers]
‣ Drum Roll to Cymbal [#46] - Martyn David, Alan Bell, Roger Stephen Dexter [drum roll again]
‣ Twelfth Street Rag - Sage Guyton and Jeremy Wakefield [Patrick dances]
‣ Sunny Samoa - Jan Rap [realtor arrives]
‣ Gator - Steve Belfer [SpongeBob pretending to be Squidward]
‣ Dramatic Cue (a) - Ronald Hanmer [Squidward sees the realtor's car]
‣ Gator - Steve Belfer ["What kind of fool do you take me for?"]
‣ Sunny Samoa - Jan Rap [ending]
- As of June 11, 2016, The Internet Movie Database (IMDB) gives this episode a rating of 8.5/10 based on 309 ratings by users. TV.com users give this episode a 8.5/10 based on 206 votes.
- "Opposite Day" was ranked #54 during the Best Day Ever event on November 9–10, 2006.
- Despite it being Tuesday and Wednesday, SpongeBob and Squidward are seen not working at the Krusty Krab, but since SpongeBob actually thinks it is Opposite Day, he doesn't go to work or they possibly had a few days off.
- It is revealed in this episode that SpongeBob knows that Squidward dislikes him.
- This episode marks the first appearance of Patty Rechid.
- This episode marks the first appearance of Squidward's Self-Portraits.
- This is the first and one of the few episodes where SpongeBob's belt buckle is shown.
- The scene where SpongeBob and Patrick change each other's colors may have inspired SpongeBob PinkPants, a pink version of the SpongeBob doll that was sold in the UK around 2006.
- When SpongeBob and Patrick are talking backwards, in reverse they are saying: "Hey, Patrick," "I give up," and "To get to the opposite side." The original closed-captions said "(speaking gibberish)"
- The scene where Patrick dances around repeating "I'm Squidward" has become popular on YouTube.
- If Squidward was acting as the opposite then he'd be acting like SpongeBob and the latter would be miserable.
- Also, Patrick would be smart instead of acting miserable.
- SpongeBob's voice and face like Squidward in a similar way to how he look like Squidward in "Ripped Pants."
- Patty Rechid was voiced by Lori Alan, who also does the voice of Pearl Krabs in the series. However, the end credits mistake her voice actor as Sirena Irwin.
- This episode along with "Nature Pants" premiered exactly two years before the September 11th terrorist attacks.
- On the Drawn Together episode "Terms of Endearment," Wooldoor Sockbat has a similar joke based on this episode.
- 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.
- In Indonesia, instead speaking backwards, SpongeBob and Patrick are saying the riddle of "Why did the chicken cross the road?"
- This episode takes place after "Sing a Song of Patrick" because the framing adds to SpongeBob's house in this episode.
- This episode takes place after "Rodeo Daze" because SpongeBob's pineapple was a real fruit in that episode, but not in this episode.
- This is the first time Gary has said something other than "Meow;" he barks like a dog instead of using his trademark cat meow.
- SpongeBob impersonates Squidward for the second time, the first time being in "Ripped Pants."
- In the Greek dub of the episode when Patrick knocks at SpongeBob's door, both SpongeBob's and Patrick's lines are muted and sound is barely found during their dialogue.
- During the "Pin the Tail on the Seahorse" scene, the tail that Squidward is holding is not a seahorse's tail. Also, the image of the seahorse already has a tail.
- When Squidward's house is seen by itself after Squidward said "Opposite" after having known how to solve the issue of SpongeBob and Patrick's success of preventing Squidward selling his house, SpongeBob and Patrick's house don't appear.
- When Squidward is rebuilding SpongeBob's house, he just started. When the view changes to SpongeBob for one second, the modelling is already finished. Or maybe Bikini Bottom time is faster then real time.
- When Squidward starts to rebuild SpongeBob's house, the lower part of his body is not colored in (which is hard to tell at first since the sky was already blue).
- When SpongeBob puts the belt around his head, the belt buckle and the piece of belt that goes through it disappears.
- 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 the first season.
- When Patty walks into Squidward's house, Patrick's house cannot be seen.
- When Patty says, "Are there anymore Squidwards I should know about?," the closed-captions said, "Are there anymore Squidwards?"
- 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.
- It's possible that SpongeBob took off his belt and put it back on his waist off screen.
- In one scene, an ear on Squidward's house has a window, then it's gone.