This party was founded by Brittany when she was in her third year. It used to be a monthly party until Hufflepuff students managed to talk their Head of House, Pomona Sprout, into allowing them to host it weekly under the promise that their schoolwork wouldn't suffer from it. Since then, she and several other Professors who have been sorted into Hufflepuff are seen in the Common Room when these parties are being held, enjoying the music, all to get away from the stress of their daily job.
To make sure that the party goes smoothly, two Prefects are appointed to keep a close eye during the festivities. This list changes every week since there are six Prefects. Pomona Sprout has an agreement with Albus Dumbledore about it: the Prefects who are watching over the party won't have to supervise the castle at night. However, the other four Prefects need to follow their strict schedule and mostly arrive at the very end of the party. Their Housemates make sure to leave some food and drinks behind for these Prefects.
Over the year, this party has gained a reputation throughout the student body. It is known that other Houses are jealous of this party and don't like that they are not allowed to attend simply because they are not sorted into Hufflepuff. Several students have tried to follow Hufflepuff students in the hope of discovering and sneaking into their Common Room but none have succeeded so far.
During dinner every Friday night, there's significantly less food on the tables of the Houses in the Great Hall because the house elves are sending most of the food to the Hufflepuff Basement, which is convenient since their Common Room is located right next to the kitchens. The entire Hufflepuff table is completely empty; the students, the Hufflepuff Professors and the Head of House are all absent.
With the help of their Head of House, the entire Common Room is transformed into a huge bed. The ground will turn into one big pillow on which the students can sleep. The blankets, which were sewn by Helga Hufflepuff herself, will be provided by the house elves as well. These blankets are enchanted so that the person lying under it will never feel the cold. This is the only night in which the boys and the girls of every year are allowed to sleep together in one room.
Most Hufflepuff students miss breakfast on Saturday morning because they are still asleep in the basement. Thankfully, the house elves have provided breakfast for the people who did manage to get up early but didn't feel like walking to the Great Hall in order to get some food.
Students from the other Houses think it's unfair that Hufflepuff gets to host this weekly party and that the house elves are working with them by providing them with all this food. However, what they don't know is that the Hufflepuffs help them prepare these meals in the kitchen (some of them even volunteer to help them on normal schooldays), a place so sacred to them that they hardly allow anyone entrance, with the exception of the Hufflepuffs, with whom the elves have a great relationship with since they don't treat them like dirt. It is thus no rare occurence if one or multipe house elve(s) grow attached to a student and appear when their name is being called. They will then obey them like they would if they had a master whose family they served.
If there's a Quidditch match in which Hufflepuff has to play on a Friday and they manage to win, the party will be even more great and festive. If they lose, it's of course sad and unfortunate for them, but knowing that they still have a fun party ahead of them doesn't ruin their mood at all.