The gospel of justification by faith alone tells us that we must accept believers who have different opinions about disputable matters, because the gospel tells us that God accepts sinners on the basis of faith. The New Testament clearly tells us to avoid certain sins. But it doesn't address every situation and every behavior, and because of that, there will be differences of opinion within Christianity. For example, if we are convinced that we should not drink wine, we should avoid wine. But we should not call all wine-drinkers sinners, nor should we separate from them.
Wine is one of the disputable matters, and so are days and foods.
These are matters for mutual tolerance, not division and hard feelings. The Lord has called us to serve, not to judge. If he has been so gracious as to include us, we must allow him to be gracious enough to include them, too. He will make the decisions and take care of his own servants. He knew that his words applied to Sabbath days just as much as they would to superstitions about the Ides of March.