- "Yeah, like a no-fun version of a Saturday. Seemed to fit."
- ―Zak Saturday
The Monday Family (named by Zak Saturday, see above) are evil doppelgangers of the Saturday family from an anti-matter universe. The only one to be totally physically identical to her real world counterpart is Drew Monday, though the evil Drew has an extremely long snakelike tongue. The other family members all have external differences- Zak Monday’s hair that is the inverted version of Zak Saturday’s, Fiskerton’s double has horns, Komodo has spots and the ability to turn into stone, Doc Monday has an eye patch over the opposite eye to Doc Saturday, and a fingerless version of the glove, and Zon’s double is shown to have eyes on her wings.
Most of the Mondays' personalities and abilities are a reverse of the Saturdays. Zak Monday was cruel and vicious, and hated his parents - his power, instead of calming cryptids, drove them insane. Drew Monday pretends to be sweet and loving to Zak, but secretly hates him (as opposed to Drew Saturday who, though she acts tough, loves her son deeply). Doc Monday is very stupid and can barely string a sentence together, where Doc Saturday is brilliant and scientific. Komodo Monday is very civilized and intelligent and can speak perfect English in a slightly Mexican accent, though ruthless and sadistic. Similarly, Fiskerton Monday hates Zak and is very vicious and savage. The only one to seem the same is Zon, though it is clear she hates the Mondays.
The Monday family seems to be controlled or run by Komodo’s evil doppelganger, and the family seems to have a love/hate relationship with Zak and Komodo Monday and each other. They are seen fighting over the Smoke Mirror while the Saturdays escape and Drew Monday seems to hate having to pretend to like Zak Saturday and has a love of hot chocolate, which is apparently nonexistent where they come from. At the end of the episode “Black Monday”, in which the Monday family was introduced, Zak and Komodo Monday are shown escaping, leaving the rest of their family in the other world. After they walk off, you see the others yelling to let them out, and Drew Monday blatantly threatens her son, even though he's out of sight and doesn't hear her.
All of the Mondays reappear in Paris is Melting. Komodo and Zak try to destroy the Saturdays' reputation. At the end of the episode, all of them are back in the mirror and Komodo and Zak are probably getting beaten by the rest of the Mondays for leaving them.
Zak Monday is once again later released by the machinations of V.V. Argost. Captured by Munya, Argost used the flute of Gilgamesh to sap his Kur powers, and his life force, from him.
The Mondays have all of the same items that the Saturdays have only like themselves they are all reversed, the most powerful of which being their Airship, which is full of weapons, and runs on darkness rather than sunlight, causing its power to be drained when out in the sun. The Mondays' ship also shoots continuous streams of red smoke or smog, while the Saturdays' Airship produces no pollution.
So far, Anti-Doyle hasn't appeared yet. Since Doyle passed from bad to good, it is possible that Anti-Doyle turned from good to bad.
It is unknown if the dopplegangers are truly called the Mondays, as this is the name given to them by Zak Saturday. It is unlikely though as in Paris is Melting, when Zak calls his counterpart Zak Monday, he replies "Is that what you call us?" It is more likely they are also known as the Saturday family, though everyone in the anti-matter universe hates them, or possibly the Anti-Saturdays.
(Only pictures of the Mondays appeared in the Saturdays' Data Banks, and Zak Monday was the only on to canonically appear in the flesh)
- Like Argost, Rani Naga and the Reptillians, The Monday Family have also tried to kill the Saturday Family.
- Since the Mondays are the reverse of the Saturdays, their names might also be reversed like Zak Saturday's reverse name could be Kaz. But when Drew Monday was yelling at Zak Monday and Komodo Monday she called them Zak and Komodo. However the Mondays could have picked up the Saturdays' names and started calling each other by them.
- Since "Monday" was suggested by Zak, it presumed their true last name is likely Saturday, the same as their counterparts.
- This Days, such as Labor Day, Columbus Day, Martin Luther King Jr. Day, Presidents' Day and Memorial Day.
- Zak's evil doppelganger is similar to Jimmy Neutron's evil clone from The Adventures of Jimmy Neutron: Boy Genius.