Romans 14:1-9 As for the one who is weak in faith, welcome him, but not to quarrel over opinions. One person believes he may eat anything, while the weak person eats only vegetables. Let not the one who eats despise the one who abstains, and let not the one who abstains pass judgment on the one who eats, for God has welcomed him. Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand.
One person esteems one day as better than another, while another esteems all days alike. Each one should be fully convinced in his own mind. The one who observes the day, observes it in honor of the Lord. The one who eats, eats in honor of the Lord, since he gives thanks to God, while the one who abstains, abstains in honor of the Lord and gives thanks to God. For none of us lives to himself, and none of us dies to himself. For if we live, we live to the Lord, and if we die, we die to the Lord. So then, whether we live or whether we die, we are the Lord's. For to this end Christ died and lived again, that he might be Lord both of the dead and of the living.
Galatians 4:8-11 Formerly, when you did not know God, you were enslaved to those that by nature are not gods. But now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more? You observe days and months and seasons and years! I am afraid I may have labored over you in vain.
Colossians 2:16-23 Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath. These are a shadow of the things to come, but the substance belongs to Christ. Let no one disqualify you, insisting on asceticism and worship of angels, going on in detail about visions, puffed up without reason by his sensuous mind, and not holding fast to the Head, from whom the whole body, nourished and knit together through its joints and ligaments, grows with a growth that is from God.
If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations— “Do not handle, Do not taste, Do not touch” (referring to things that all perish as they are used)—according to human precepts and teachings? These have indeed an appearance of wisdom in promoting self-made religion and asceticism and severity to the body, but they are of no value in stopping the indulgence of the flesh.
Well actually on this point I agree that these verses are relevant to the discussion, and rebut those who claim that Christians MUST worship on a certain day. Good reply!