That is the question traditionally asked by the youngest child in a Jewish family as they prepare for the Seder. What is the Seder? It is a remembrance mandated by God that His People remember the Passover - which brought about the freeing of the Israelites from Egypt. It was that same Seder which Jesus was celebrating with His Apostles on what we now call Holy Thursday - or more traditionally: Maundy Thursday. Maundy comes to us from the Latin word mandatum, which is also the root for the English word "mandate."
The New Mandate
When Jesus Christ celebrated the First Eucharist - it was not to be a one-time deal. He commanded, or mandated, that they were to "do this" whenever they partook in what we now call the Eucharist. He essentially ordains the Apostles to carry on this tradition and as they were sent out in the same manner Jesus was sent by the Father - they too, in turn, were to ordain others to perpetuate this New Mandate.
Today is the day we remember that Jesus Christ offered Himself wholly, body and spirit, to redeem us from our sins. This is why EVERY Friday we are mandated to offer up meat, or something equivalent (meat is still preferred by many) on EVERY Friday throughout the year, not just Good Friday. Why? Because EVERY Friday is like a little Good Friday and EVERY Sunday is like a little Easter Sunday. EVERY Friday is a special time to offer up a small penance to recall Jesus' passion and death just as EVERY Sunday is a special time to recall His resurrection and victory over death.