A few days ago, I suggested in no great seriousness that the Visitation could be considered the birthday of the Church. I was surprised both by the energy with which this suggestion was resisted and by the persuasiveness with which Pentecost as Church's Birthday was argued. My surprise was mostly due to the fact that the statement, "Pentecost is the birthday of the Church," always struck me as too twee to be particularly meaningful.
Well, I know a little better now, thanks to those who responded, and was able to listen to the homily today (by a permanent deacon, one of fifteen, ordained yesterday for the Archdiocese of Washington) without disengaging when it opened with a proclamation that we were celebrating our birthday today.
"No birthday is complete without a gift," the deacon preached, and of course on Pentecost the gift was the Holy Spirit. A comfortably Thomistic notion from one so newly ordained; St. Thomas taught that "Gift, taken personally in God, is the proper name of the Holy Ghost."
It's also interesting (to me) that Pentecost is a birthday because, and only because, a birthday gift was given. And God said, "Happy Birthday!," and it was a birthday. God is sovereign that way.