- 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 decides to move out of Bikini Bottom the next day. That afternoon, he calls Patty Rechid from Bikini Realty to arrange for his house to be sold, so that he may move out of the neighborhood. 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.A now-goofy Squidward tells SpongeBob that it is Opposite Day the next morning, and that everyone must act opposite to how they usually act. A now-grumpy 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 angrily storms out. Squidward begs Patty to sell his house, but she refuses stating she wouldn't sell the house if he was the "last Squidward to exist 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" (likes) 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. SpongeBob then asks Patrick if he ever feels that Squidward likes them too much.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ Hawaiian Link (b) - Richard Myhill [Squidward sleeping]
‣ Sunny Samoa - George de Fretes, 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/SpongeBob and Patrick enter Squidward's house]
‣ 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 - George de Fretes, Jan Rap [ending]
- This episode is available on the Tide and Seek, Complete 1st Season, First 100 Episodes, and Hall Monitor DVDs.
- As of February 1, 2019, The Internet Movie Database (IMDB) gives this episode a rating of 8.6/10 based on 541 ratings by users. TV.com users give this episode a rating of 8.5/10 based on 209 votes.
- "Opposite Day" was ranked #54 during the Best Day Ever event on November 9–10, 2006.
- This episode premiered in Canada on July 15, 2000.
- Despite it being Tuesday then Wednesday, SpongeBob and Squidward are seen not working at the Krusty Krab.
- 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 plush doll.
- 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)."
- This gag is a reference to the well-known joke "Why did the chicken cross the road?"
- The scene where Patrick dances around repeating "I'm Squidward" has become a popular meme on YouTube.
- On the Drawn Together episode "Terms of Endearment," Wooldoor Sockbat has a similar joke based on this episode.
- This is the first episode where Gary has said something other than "Meow"; he barks like a dog instead of using his trademark cat meow.
- This episode marks the first appearance of Squidward's self-portraits.
- This is the first episode where SpongeBob's belt buckle is shown.
- This is the second time SpongeBob impersonates Squidward. The first time is in "Ripped Pants."
- 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!," which translates to "Who were we? Bubbles. We blew ourselves and popped ourselves!" The line isn't spoken backwards.
- Also, when SpongeBob shouts "I'm not ready!," he just laughs.
- In Indonesia, instead speaking backwards, SpongeBob and Patrick are saying the riddle of "Why did the chicken cross the road?"
- 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," SpongeBob and Patrick's house don't appear. Also, in that same scene, an ear on Squidward's house has a window, then it disappears.
- When Squidward starts to rebuild SpongeBob's house, the lower part of his body is not colored in.
- When SpongeBob puts the belt around his head, the 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.