Why Do Some Churches Say You Need to Keep the Mosaic Law?

Questions about why some churches say you need to keep the Mosaic Law and the gospel of Christ to be saved, and whether or not it’s inappropriate for Christians to celebrate Passover since it’s a celebration from Judaism and the Messiah has already come. Why do some churches say you need to keep the Mosaic Law and the gospel of Christ to be saved? Is it inappropriate for a Christian to celebrate Passover since it’s a celebration from Judaism and the Messiah has already come?