Exodus 5:1 reveals what Moses and Aaron said to Pharaoh when they first met with him:
Exodus 5:1 And afterward Moses and Aaron went in, and told Pharaoh, Thus saith the LORD God of Israel, Let my people go, that they may hold a feast unto me in the wilderness.
When, however, we examine all the preceding statements in Exodus 3-4 about what God told them to say to Pharaoh, we do not find anything about His telling them to tell Pharaoh about their celebrating a feast to the Lord in the wilderness:
Exodus 3:18 And they shall hearken to thy voice: and thou shalt come, thou and the elders of Israel, unto the king of Egypt, and ye shall say unto him, The LORD God of the Hebrews hath met with us: and now let us go, we beseech thee, three days’ journey into the wilderness, that we may sacrifice to the LORD our God.
Exodus 4:22 And thou shalt say unto Pharaoh, Thus saith the LORD, Israel is my son, even my firstborn: 23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.
How, then, do we explain what Moses and Aaron said to Pharaoh about celebrating a feast to Him in the wilderness?