Colossians 2:16-17
16 Therefore, let no one pass judgement on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath 17 These are a shadow of the things to come, but the substance belongs to Christ.

The Old Testament is full of instructions about food and festivals (among other things). The people are told which animals and clean and unclean for food.  The new moon festival refers to the first day of the month when they were required to bring their sacrifices and burnt offerings to the Lord. The Sabbath was a day of rest when they weren’t allowed to do any work. 

All these things were set up by God but they were meant to put the attention on to Him, not to themselves become little "gods" to be served.  In Isaiah 1:13-14 God says:
"Bring no more vain offerings; incense is an abomination to me. New moon and Sabbath and the calling of convocations— I cannot endure iniquity and solemn assembly. Your new moons and your appointed feasts my soul hates; they have become a burden to me; I am weary of bearing them."

The people were doing things that God had told them to do but He says that he hates them! It’s obviously possible to follow the rules but completely miss the point. Paul is drawing attention to the fact that all those things are just a shadow, the substance is Jesus.
Things that may have been designed to glorify Jesus can end up taking His place.Worship Jesus, not the shadow