Moses appointed certain seasons of memorial; such, for example, as the passover; Exodus 12:23-24.
Related Commentaries of Psalms 81:4
Psalms 81:4-5
4 For this was a statute for Israel, and a law of the God of Jacob.
5 This he ordained in Joseph for a testimony, when he went out through the land of Egypt: where I heard a language that I understood not.