Luke 4:16. He went into the synagogue on the sabbath day, and stood up for to read.
When the law and the prophets were read in the synagogue those who read were expected to stand. See note on Acts 13:15 (#834). Not only priests and Levites but common Israelites were allowed to read the Scriptures publicly. Every Sabbath seven persons read: a priest, a Levite, and five ordinary Israelites.