Good Friday (also called Black Friday) is a Christian holiday. Christians remember on that day that Jesus Christ died on the cross in 33 AD.
It is celebrated on the Friday before Easter, sometime in March or April. Easter celebrates the resurrection of Jesus. It is called Good Friday because Jesus conquered death.
But it is also called Black Friday because Jesus died for all the sin of mankind, and the destructive power of sin and death became very obvious.
In the US, Black Friday is the Friday after Thanksgiving (fourth Thursday in November), and it is a commercial event where people are urged to spend money on special offers. Thanksgiving is a public holiday in the US and Canada, where the families meet and celebrate the harvest. It is called Black Friday, because the retailers make good profit, and use black ink instead of red ink for writing down their profit. It is the start of the shopping season before Christmas, which originally celebrates the birth of Jesus Christ.