What does the date Friday the 13th have to do with Christianity?

Friday the 13th has nothing to do with Christianity.
Because Jesus Christ was believed to have been crucified on Friday, the day before the Jewish Sabbath, and because there were thirteen people present at the Last Supper (Jesus and the 12 disciples), some people made the connection that linking Friday and 13 together meant bad luck. It is purely a superstition, and has nothing whatsoever to do with Christianity.

The superstition gained more ground when Philip IV of France ordered e arrest of all of the Knights Templar in France on 13 October 1307, which just so happened to be a Friday. The knights were put on trial, tortured and executed. They had many sympathisers, and through folklore which was handed down through the generations, the part that Friday the 13th had to play in the events became imbued with superstitious horror and a sense of evil being attached to the day. Once again, there was, and remains, no link between Friday the 13th and Christianity. It is all to do with folklore and man's natural tendency to be superstitious.