Pentecost is often called the Church's birthday, but a case can be made for the Feast of the Visitation, too.
At Pentecost, the disciples "were all filled with the Holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim." It is this combination of the presence of the Holy Spirit and the public proclamation of the Gospel that marks the public beginning of the Church.
A few decades before, "Elizabeth, filled with the holy Spirit, cried out in a loud voice," proclaiming as blessed she "who believed that what was spoken to you by the Lord would be fulfilled." The presence of the Holy Spirit, and a proclamation of the Gospel -- though, admittedly, not in public.
But what did happen for the first time at the Visitation is the coming together of a community united by the Spirit of Christ, Emmanuel, Who was present among them.
The Monastery of the Visitation posted a wonderful reflection on how the sisters there experience this feast -- for them, a solemnity -- as a community of Marys and Elizabeths.