In the New Testament is when the Holy Spirit descended upon 120 followers of Jesus Christ and is traditionally called the Spiritual Birthday of the Christian Church.
And Jesus came and spake unto them, saying, All power is given unto Me in heaven and in earth. (19) Go ye there fore and teach all nations, baptizing them in the name of the Father, and the Son and of the Holy Ghost: (20 ) Teaching them to observe all things what so ever I have commanded you: and, lo. I am with you always, even unto the end of the world. Amen. ( Matthew 28:18-20)