Well; it's in the BOOK a couple of places...
Romans 14:5 (niv)
One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind.
Acts 15
To the Gentile believers in Antioch, Syria and Cilicia:
Greetings.
24 We have heard that some went out from us without our authorization and disturbed you, troubling your minds by what they said. 25 So we all agreed to choose some men and send them to you with our dear friends Barnabas and Paul— 26 men who have risked their lives for the name of our Lord Jesus Christ. 27 Therefore we are sending Judas and Silas to confirm by word of mouth what we are writing. 28 It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements: 29 You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things.
Farewell.
In Romans 15:5 Paul was addressing Romans who had converted to Christianity. He was talking about the Jewish ceremonial sabbaths or holy days that came on a yearly basis. The newly converted Roman Christians were not required to keep them. As for Acts 15 I have no idea how you can get Sunday worship out of that.