So, let's imagine we are kids playing in a playground. There are two groups of kids, let's call them the Red team and the Blue team. One day, the grown-ups who are in charge of the playground decide that it's time for the Red and Blue teams to stop playing against each other because they were arguing a lot and sometimes they even fought!
The grown-ups organize a big party for all the kids in the playground. Everyone is happy, they dance, eat candy, and sing songs. But then, something terrible happens. Suddenly, some of the grown-ups pull out their weapons and start attacking one team, the Red team. The other kids try to run away, but the grown-ups keep chasing and killing them.
This is what happened on St. Bartholomew's Day in France in 1572. The grown-ups who attacked were actually soldiers and they were sent by the powerful people who wanted to get rid of the Red team, who were Protestants. The powerful people who organized the attack were Catholics and they didn't like the Protestants because they had different beliefs about God.
So, to sum it up, the St. Bartholomew's Day Massacre was a terrible event in which many Protestants were killed by soldiers sent by the Catholics in France, just because they had different beliefs about God. It was a very sad day and it's important for us to remember it and learn from it, so we can all live peacefully together, no matter what we believe in.