On the sabbath day— In the Greek it is plural, τοις σαββασιν, and, when compared with the parallel place in St. Luke, shews that it was the custom of Jesus to frequent these places of worship on the sabbath days. The addition of this circumstance seems therefore intended to intimate, that Christ was free from the imputation of profaning the sabbath, which was objected to him by the Jews. See Matthew 12:1-2 and Grotius. Dr. Heylin renders the latter part of this verse, where he immediately began to instruct publicly in the synagogues, to which he went on the sabbath day.
Related Commentaries of Mark 1:21
Mark 1:21
21 And they went into Capernaum; and straightway on the sabbath day he entered into the synagogue, and taught.