Peter’s reference to holiness comes from the book of Leviticus. This was an O.T. book that served as a manual for the Levites who were to guide the people in the ways of worship. You will find the word holy over 70 times in the 27 chapters. The Israelites were chosen by God to be a holy nation among the nations of the then known world. They were to serve only One God and their daily lives were to reflect the holiness of the God they served.
The laws of the book of Leviticus plus the various feasts were done away with when Christ came. He fulfilled all the feasts and rituals. I Corinthians 5:7 clearly tells us that we are to “Get rid of the old yeast
(O.T. rituals)... For Christ our Passover lamb has been sacrificed.”
The holiness Peter speaks of in this verse does not come from ritual, ceremonials or religious feasts. It comes from a personal, daily walk with Christ who is holy and infuses us with His holiness. Let me quote verses 15 and 16. “Just as He who called you is holy, so be holy in all you do; for it is written: Be holy because I am holy.”