What event was important to the beginning Christianity ​

History of Christianity
According to the Bible, the first church organized itself 50 days after Jesus's death on the Day of Pentecost—when the Holy Spirit was said to descend onto Jesus's followers. Most of the first Christians were Jewish converts, and the church was centered in Jerusalem.

Advent Sunday. Advent Sunday is the start of the Christian year and, in the Western churches, is four Sundays before Christmas.