Acts 18:19

And he came to Ephesus, and left them there: but he himself entered into the synagogue, and reasoned with the Jews.

This was Paul's first brief visit to Ephesus, made just after leaving Corinth while traveling on to Jerusalem for a festival. The festival he was going up to keep was Pentecost.

He had brought Aquila and Priscilla with him to Ephesus and left them there to carry forward the good work he had begun. He himself entered into the synagogue and reasoned with the Jews.

His teaching made such a favorable impression that the Jews entreated him to stay and continue his labors among them. He could not tarry because he had to continue his journey to Jerusalem, but he promised to return and labor with them if God willed.

The same setting later became the place where Apollos spoke boldly and Aquila and Priscilla helped him understand the gospel more fully. This arrival began a connection with a three-year evangelistic ministry in the region, during which he worked at his trade and was accompanied by Aquila and Priscilla, who had traveled with him back to Asia.