When I was growing up (yes I know it was some time ago) I don’t recall people talking about Pentecost, but rather Whitsunday. Fifty days after Easter we recall the coming of the Holy Spirit, filling the shattered disciples with a new energy and courage.
It seems to have become quite fashionable to declare Pentecost as the Churches birthday, I guess the argument is attractive and easy, as this represents the first day the Apostle went out and declared the message of the resurrection publicly. It is a quite easy way to avoid a more serious consideration of what the gift of the Spirit should mean,
I actually wonder if this approach is not in fact theologically inadequate. We are the Church, We are the Body of Christ, and as such should we not perhaps think of Christmas as the Church’s birthday, or given the eternal nature of the Godhead, perhaps we don’t have a birthday at all.