Acts 18
20 When they asked him to stay a longer time with them, he did not consent, 21 but took leave of them, saying, “I[f] must by all means keep this coming feast in Jerusalem; but I will return again to you, God willing.” And he sailed from Ephesus.

So to clarify, Paul had to leave so he could attend a feast in Jerusalem.
Some commentary:
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.

Other people comment that this shows that as Christians we should STILL be recognizing Jewish holidays. I’m not really sure I would see that. I merely see this as the Lord giving us more insight into Paul’s character.