Afterward Moses and Aaron went and said to Pharaoh, "Thus says the LORD, the God of Israel, 'Let my people go, that they may hold a feast to me in the wilderness.'"
Aaron was a concession from God to Moses. Moses did not trust that God would give him the voice and words required when he faced Pharaoh with God's command to let Israel go. So God allowed Aaron to speak on Moses behalf.
It is an enormous thing to claim to have words from God himself. Every command and every piece of wisdom is absolute. It is common for a prophet to begin a message from God with "Thus says the LORD" to show the gravity of the message. LORD in capitals means that the actual name of God was used but out of reverence it was not recorded in writing or spoken aloud.
A simple feast in the wilderness was never going to happen. It seems that this was not a good enough reason to let Israel depart peacefully. Pharaoh would not easily let them go and God would bring plagues upon Egypt until Pharaoh finally relented.