The days of the week in Chowder consist of seven days total, however have different names than the ones in real life. The days of the week in the series are named after numbers, presumably starting from and ending from Onesday to Sevensday before a new week begins.
Names of the days of the week
Onesday, Twosday, & Sevensday
These days of the week haven't been mentioned in the show at all, however their existence is implied by the days of the week already mentioned within the show.
Threesday is only briefly mentioned in the puppet sequence during the credits for "The Vacation" and "The Sleep Eater". It's the day when Mung Daal planned to show Chowder how Ninny Nanny Nougat is made.
Foursday is never mentioned in the show, and its existence is only implied by the already-existing days of the week.
Fivesday is the day in which Shnitzel gets his weekly pay as stated in "Shnitzel Makes a Deposit" and is also the day "The Hot Date" takes place on. The most common day of the week for weekly and bi-weekly payments is Friday, which is why many presume Fivesday falls on a Friday.