Friday the 13th is a day that happens when the date on the calendar is both Friday and the number is 13. Some people believe that this day is very unlucky and bad things can happen to you.
A long, long time ago, there was a myth or story that there were 13 people who sat down at a table to eat, and one of them was really, really bad. It was said that this person caused lots of trouble and did really bad things. The person who this story was about was Judas, one of the 12 disciples of Jesus, who later betrayed him. Because of this story, people believed that 13 was an unlucky number.
Also, Friday the 13th is associated with bad luck and scary things because it is believed that Jesus was crucified on a Friday, and in some stories, there were 13 people present during the Last Supper the night before he was crucified.
People have different beliefs about Friday the 13th. Some people are really scared of this day and don't do anything new or important on this day. They may not even leave their house! Other people don't believe that Friday the 13th is unlucky and just treat it like any other day.
So, Friday the 13th is just a day on the calendar that some people believe brings bad luck, but not everyone thinks this way!