Romans 14:1 As for the one who is weak in faith, welcome him, but not to quarrel over opinions.
The key to understanding Romans is the first verse. If you didn't know whether meat at a community meal had been offered to idols and chose to eat only vegetables to stay on the safe side, then that was your opinion. If you felt free to everything at a community meal, then that was your opinion. However, if you felt free to disregard God's dietary laws, than that was disobedience to God.
Luke 18:11-12 The Pharisee stood and prayed thus with himself, ‘God, I thank You that I am not like other men—extortioners, unjust, adulterers, or even as this tax collector. 12 I fast twice a week; I give tithes of all that I possess.’
Didache 8:1 Your fasts should not be with the hypocrites, for they fast on Mondays and Thursdays. You should fast on Wednesdays and Fridays.
Romans 14:6 makes it clear that regarding one day as more sacred than another is in regard to fasting. Fasting twice a week was a common practice in the 1st century and was often done to commemorate certain days, but it was a matter of opinion. Like the Pharisees and in the Didache, people were judging each other according whether they fasted and when they fasted, but the only time God commanded people to fast was Yom Kippur. So again, Paul was talking about disputable matters of opinion, not about obedience to the commands of God.
There had differences of opinion about how to keep the Sabbath, but they didn't have differences of opinion about whether to keep it. The Sabbath is not even mentioned in Romans 14.