I will visit you after passing through Macedonia, for I intend to pass through Macedonia,  and perhaps I will stay with you or even spend the winter, so that you may help me on my journey, wherever I go.  For I do not want to see you now just in passing. I hope to spend some time with you, if the Lord permits.  But I will stay in Ephesus until Pentecost,  for a wide door for effective work has opened to me, and there are many adversaries. 1 Cor. 16:5-9 (ESV)
Paul knows that the Corinthians need him to be there for sometime. One Sunday sermon, one letter is not going to effect lasting and stable change in the congregation. They need pastoral care, and that takes time. But he wants to stay in Ephesus until Pentecost.
Pentecost was a Jewish holiday, marked by fifty days after the Passover. Yet it is mentioned here in a Christian context, to a congregation comprised mostly of gentiles. Here we see already the beginning of Christian time, the seasons of the Church, being separated from worldly, secular time. The Church year as we now have it may not have been formally utilized by this time. But we see that to some extent even the earliest of Christians marked time differently then those around them. Paul tells them he plans to stay in Ephesus until Pentecost, because a wide door of effective work has opened for him, the problems in Corinth will have to wait. Paul also mentions that there are many adversaries. The Gospel will always be opposed. Where the Gospel is being most effective the Devil will train his troops, and amass his forces. They will attack from the outside, and inside. Where ever the adversaries of the Gospel are to be found they must needs be opposed. There will be fighting, the church on earth will never know peace as long as it holds to the gospel. For this reason it is the Church Militant, its sword the word of God.