God told Moses that the Passover should be celebrated through all time. Twelve hundred and fifty years later Jesus celebrated the Passover at the Last Supper. Our Mass is a celebration of the Passover.
Now, while we identify the Passover with that last meal of the Hebrews before their leaving Egypt, the term was broadened to take in two farther Passovers: the passing through the Red Sea, and the passing over the Jordan into the Promised Land.
When we refer to this as the Paschal Season, it is all three Passovers combined that we that we are re-enacting.