I have no problem with another brother who is observing a particular day ect . Honestly I would class the person as a weaker brother in this area. The issue is when someone say you must observe the sabbath .
col 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
Gal 4
9But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?
10Ye observe days, and months, and times, and years.
11I am afraid of you, lest I have bestowed upon you labour in vain.