Gryffindor:

The Gryffindor students have the loudest celebrationsThe day starts with all the other students in their dormitory singing happy birthdayDuring the day the students will randomly burst into song, and they can be heard singing everywhere in the castleThe birthday girl/boy/person is sometimes given a crown or hat to wear during the day, and every time they see someone wearing one, they all start singingOnce, Fred and George Weasley enchanted the statues and suits of armour in the school to sing every time she/he/they walked byThe singing always drives the professors crazy, and all except Dumbledore, Lupin and Hagrid give out detentions to anyone who sings in their class "You're SO OLD!!"

Ravenclaw:

Every birthday the birthday girl/boy/person has to complete a treasure hunt, and are given clues and riddles to find the treasure throughout the dayThe clues are given written down on pieces of paper or in other magical waysEach riddle leads to another clue and they eventually lead to the treasure that is hidden somewhere on the grounds or in the castleThe treasure varies for each student and is personalised for that specific person by their friendsThe students of Ravenclaw always makes sure to prepare small but clever surprises for birthday girl/boy/person to discover throughout the day, all leading up to the big gift

Hufflepuff:

On their birthdays, the Hufflepuffs get woken up with cake in bed before breakfastThe night before the other students prepare the celebration and the cake, and they visit the kitchen for ingredientsThe Hufflepuffs make sure to never forget any birthday, and it's a priority to celebrate every birthday equally much, even for students who are not their closest friendsThe students who have their birthday in summer or during the Christmas break gets celebrated when they come back in autumn, no one is forgottenThe student is presented with little surprises throughout the day, like all the portraits wishing them a happy birthday

Slytherin:

The Slytherins plan their birthday celebrations for a long time, with organised teams within the house for each part of the celebrations, like music, cake, decorations etc.It's important for the Slytherins to throw the perfect birthday celebrations, and the students pride themselves in having the best parties and surprisesEvery student gets their own little party, sometimes with only their closest friends and sometimes with almost every student of the houseThe parties are often supposed to be surprisesBut the birthday girl/boy/person almost always figures it out beforehandEvery student gets a personalised verse of happy birthday or their own short song, prepared by the students in the "song group"