They were keeping it "unleavened "ceremonially, and he urges a spiritual unleavening as more important. The Christian hallowing of Pentecost connects with the Paschal argument.
As we should expect, he tells them all exactly, informing them as friends. For this too is a mark of friendship to say the reason why he was not with them, why he delayed, and where he was staying.
[AD 196] Polycrates Of Ephesus on 1 Corinthians 16:8