Yesterday, although it was a feast-day of Satan, you preferred to keep a spiritual feast, recieving our words with great good will, and spending most of the day here in church, drinking a drunkness of self-control, and dancing in the chorus of Paul. – On Wealth and Poverty, Sermon 1

The feast day that John mentions is Saturnalia. Many have painted the corruption of the Church with the brush of conspiracy, but here we see that it was not conspiracy, but a matter of people choosing to hold a worship servicce as opposed to partaking in the feast of Saturnalia.