Colossians 2:16 Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: 17 Which are a shadow of things to come; but the body is of Christ.
Under the Old Covenant and the Law given through Moses the Jews were required to observe certain days.
Exodus 23:14 Three times thou shalt keep a feast unto me in the year. 15 Thou shalt keep the feast of unleavened bread: (thou shalt eat unleavened bread seven days, as I commanded thee, in the time appointed of the month Abib; for in it thou camest out from Egypt: and none shall appear before me empty:)16 And the feast of harvest, the firstfruits of thy labours, which thou hast sown in the field: and the feast of ingathering, which is in the end of the year, when thou hast gathered in thy labours out of the field. 17 Three times in the year all thy males shall appear before the Lord GOD.
Jesus fulfilled the Law and with his death and resurrection inaugurated the New Covenant.
The Law given through Moses is no longer in force.
Therefore, those baptised into Christ are not required to observe the feast (holy) days.
They are not to be judged or condemned for not observing the days.
The holy days of the Law were a ‘shadow’ of ‘things to come’.
Colossians 2:17 Which are a shadow of things to come; but the body is of Christ.
The Law was ‘schoolmaster’ to bring us to Christ.
Galatians 3:24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. 25 But after that faith is come, we are no longer under a schoolmaster.
Under the New Covenant in Jesus Christ there is only one observance required and that is to meet often with fellow believers to share bread and wine in remembrance of his sacrifice. No day is specified.
First century believers found it convenient to meet on the first day of the week and this became the tradition.
1 Corinthians 11:23 For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25 After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26 For as often as ye eat this bread, and drink this cup, ye do shew the Lord’s death till he come.
This is not to be confused with the Sabbath (the seventh day) which is no longer to be observed.
I hope you find this helpful.