1 Corinthians 16:8

The Jews celebrated Pentecost in late May or early June so Paul probably wrote 1 Corinthians in the spring of the year (cf. 1 Corinthians 5:7; 1 Corinthians 15:20). It is not unusual that since he was a Jewish believer with the evangelization of Jews on his heart he would refer to important events in the Jewish calendar such as Pentecost (Leviticus 23:15-21). Perhaps the early Christians paid more attention to the significant events in the life of the church than many churches do today. Churches that observe "the Christian year" tend to make more of these observances. The feast of Pentecost, of course, also marked the coming of the Holy Spirit (Acts 2).