Commentaries:
Here we have a mention of a holy day, which may have been the Feast of Tabernacles. Paul says, "I must by all means keep this feast." The Protestant interpretation of the book of Galatians primarily, and the book of Romans secondarily, puts the apostle Paul into the position of being a hypocrite! These commentators suggest that he tells people, on the one hand, that they do not have to keep the law of God, the Sabbath, and the holy days, yet the book of Acts shows him in every city keeping the Sabbath and here telling the people, "I must keep this feast." They make him out to say one thing but do another.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty-Four)
Other Forerunner Commentary entries containing Acts 18:21:
Galatians 4:9-10
Galatians 4:12