Exodus 10– If the Feast Days were not yet known, why did Moses say they were going to celebrate one?

Exo 10:8
So Moses and Aaron were brought back to Pharaoh, and he said to them, "Go, serve the LORD your God! Who are the ones that are going?"

Exo 10:9
And Moses said, "We shall go with our young and our old; with our sons and our daughters, with our flocks and our herds we will go, for we must hold a feast to the LORD."

If the Feast Days were not yet known, why did Moses say they were going to celebrate one? This was already mentioned in Ex. 5:1. While many of the Feast Days may or may not yet have been fully defined, they were already part of the Hebrew culture and Law.