サプライズパーティーって英語でなんて言うの？

If you are going to have a party for your friend John and he does not know about it, you can say, "We're having a surprise party for John" or "We are going to have a surprise party for John". If it is his birthday you can say, "We're giving John a birthday surprise and having a party".

example "I want to throw a surprise party for Angela". or "can you help me plan a surprise party". or "the party is a surprise, please keep it secret!".

A surprise party, is a party that is being held for a person, but the planning of it has been kept a secret from the person it is for. Usually, the friends and family of the person will gather together and wait for the person to arrive, and when the person enters the room everyone will shout "SURPRISE!"

The best word to explains a party that is a secret for a person who does not know about it is called a "Surprise Party"

It's a surprise and it's a party. So a party that is a surprise is called 'a surprise party ' You could call it a surprise celebration if it is just drinks or a small gathering of people.

A party where the guest of honor doesn't know there will be a party hosted in their honor, then you call that a surprise party. For example, you can say: -My family throws me surprise parties every year for my birthday.