Adam Clarke on Acts 18:21

I must - keep this feast - Most likely the passover, at which he wished to attend for the purpose of seeing many of his friends, and having the most favorable opportunity to preach the Gospel to thousands who would attend at Jerusalem on that occasion. The whole of this clause, I must by all means keep this feast that cometh in Jerusalem, is wanting in ABE, six others; with the Coptic, Ethiopic, Armenian, and Vulgate. Griesbach leaves it in the text, with the mark of doubtfulness; and Professor White, in his Crisews, says, probabiliter delenda. Without this clause the verse will read thus: But he bade them farewell, saying, I will return again unto you, if God will. And this he did before the expiration of that same year, Act 19:1, and spent three years with them, Act 20:31, extending and establishing the Church at that place.