Okay kiddo, have you ever heard of a thing called a "massacre"? It's where a lot of people are hurt or killed in a very violent way.
So now, let's talk about something called the "St. Brice's Day Massacre". This happened a long time ago, back in the year 1002 in a place called England.
At the time, people who believed in a different religion than most of the people in England were being treated pretty badly. They were called "Danish Vikings" because they came from a place called Denmark.
The leader of England, a man named King Aethelred, was worried that the Danish Vikings would try to take over his country. So he ordered all of the Danish Vikings who were living in England to be killed on St. Brice's Day, which is a holiday in England.
This was a really bad idea because a lot of innocent people who weren't even Danish Vikings were hurt or killed too. And even though the king tried to stop the massacre later, it was too late.
So that's what happened during the St. Brice's Day Massacre - a lot of people were hurt or killed because of their religion and where they came from, and it was a very sad time in history.