At the time Paul wrote to the people of Colossae, the importance of following certain practices or customs was widely taught as being crucial in following Christ. There was Passover to be followed, the Feasts of the Unleavened Bread, Pentecost, the Day of Atonement, the Feast of Tabernacles. There were foods that were prohibited and burnt offerings to be offered to God, along with holidays that must be observed.
If you were not partaking in these you could be judged harshly by those that were partaking. Many were under the belief that if you didn't follow these practices that you could never be right with God.
This is what Paul is addressing when he tells us in verse 16, "... do not let anyone judge you...". If we look back to Colossians 2:15, Paul tells us of Jesus, "..triumphing over them by the cross." It is because of His sacrifice on the cross that we should not fear being judged by others. It is because Jesus fought the battle for us on the cross and victoriously won that we no longer must worry about the laws of the Old Testament.
Now we have a choice as to whether or not we follow those laws of the Old Testament. None of them should ever be considered "bad". They served a great purpose, but once Jesus came to this earth they were no longer necessary. He took the place of any need to follow the laws of the Old Testament. And, while there is nothing wrong with following any of these laws, there is NO NEED TO FOLLOW THESE LAWS.
Because of Jesus, ALL our days belong to God -- not just the certain holiday that once had to be observed. Because of Jesus, ALL foods are pure -- not just certain ones that have been deemed acceptable.
Because of Jesus, there is no need to worry about all the "do nots". As stated in the Enduring Word on-line commentary, "Christianity is a moral religion; it does have clear moral boundaries. But at its foundation, Christianity is a religion of positive action." Because of the cross, we don't have to worry about what we cannot do but rather what we can do.