- Scallops and clams
- Squidward Tentacles
- SpongeBob SquarePants
- Patrick Star
- Seahorse (debut; on a poster)
- Incidental 49A (debut)
- Gary the Snail
The episode starts by showing Squidward 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 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, the real estate agent 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 the realtor, who, upon hearing that he is "Squidward" as well, finally sees through everything and berates 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, which it's the last straw and she angrily storms out and officially declined Squidward's house of being sold. Squidward begs the realtor 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 "likes" (hates) them by attempting to run them over with the 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 before the episode ends.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ Grass Skirts Blowing (Hilo March) - Jan Rap, The Hawaiian Serenaders [Title card]
‣ 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]
• Nick's Perc. O.D - Nicolas Carr [Overlays "Duncan Gray"]
• 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 - George de Fretes, 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]
• 12th Street Rag - Sage Guyton, Jeremy Wakefield [Patrick dances]
‣ Sunny Samoa - George de Fretes, Jan Rap [realtor arrives]
• Seaweed 1 - Steve Belfer [SpongeBob pretending to be Squidward]
‣ Dramatic Cue (A) - Ronald Hanmer [Squidward sees the realtor's car]
• Clarinet - Brad Carow [SpongeBob playing clarinet]
• Seaweed 1 - 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, Hall Monitor, Laugh Your Pants Off, Sponge Crazy, First 100 Episodes, SpongeBob SquarePants, iSponge Offer Pack, The Ultimate SpongeBob Box Set, SpongeBob SquarePants Vol. 1, The Complete First Season, From the Beginning, Part 1, The SpongeBob SquarePants Collection, Back to Nature!, The SpongeBob SquarePants 8 Season DVD Collection, The First & Second Seasons, Movies & TV Collection, and The Best 200 Episodes Ever DVDs along with Nicktoons Collection Volume 2 on GBA Video and on VideoNow.
- This episode is available on the Laugh Your Pants Off and Tide and Seek VHS tapes.
- It is also on the Polish version of the Halloween DVD.
- "Opposite Day" was ranked #54 during the Best Day Ever event from November 9–10, 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)."
- This gag is a reference to the well-known joke "Why did the chicken cross the road?"
- 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.
- A portion of the episode was used in the 2000 Kids' Choice Awards, but redubbed with new dialogue and a kids' choice green screen background.
- This is the first episode to use the APM track "Sunny Samoa."
- This is the first episode where SpongeBob's belt buckle is shown.
- This is one of the rare times in season 1 where Squidward cries.
- This also happens in "Help Wanted."
- On top of that, this is one of the rare times in episodes where both Squidward and Patrick cry, with others including "Best Day Ever", "Atlantis SquarePantis" (with Squidward having tears of joy), "What Ever Happened to SpongeBob?," "Truth or Square," "The Fish Bowl," and "The Clam Whisperer." Other certain episodes either have Squidward or Patrick crying, depending on the episode.
- This is the second time SpongeBob impersonates Squidward. The first time is in "Ripped Pants."
- Squidward chasing SpongeBob and Patrick with a bulldozer is actually shows how much he hates them, which he really does hate them a lot. The exact opposite would be Squidward doing something nice to them likes throwing them a party and/or giving them gifts.
- 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 the Bahasa Indonesia dub, instead of speaking backwards, SpongeBob and Patrick are telling the riddle: "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.
- This and its' paired episode "Nature Pants" both premiered on Nickelodeon the same day Looney Tunes on Nickelodeon had its' final run on the network.
- 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 SpongeBob and Patrick leave the house, the green piece of confetti becomes blue when it lands on the ground.
- When Squidward's house is seen by itself after Squidward says, "Opposite," SpongeBob and Patrick's houses 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.
- 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.