On the day of Shavuot. For the thousands that came to be followers of Yeshua on that eventful Shavuot were not added to the Christian Church (which was not yet in existence), but to the followers of The Way (so the disciples of Yeshua were called, cf. Acts 9:2; 19:9, 23; 24:14, 22), a sect of Judaism. Nor did they see themselves as some new entity, or some new beginning. Rather, they saw the revival of faith among Jews as well as the ingathering of the Gentiles as evidence of the faithfulness of God to Israel, keeping His covenant promises to make Israel a light to the nations, and to bless the nations by bringing them to that light. Therefore the Messianic movements started at Acts 2. And has always been a thread of Messianic from that time till now.