Why are there TWELVE days of Christmas? When (date and hour) do they officially commence?

"The 12 days of Christmas is the period in Christian theology that marks the span between the birth of Christ and the coming of the Magi, the three wise men. It begins on December 25 (Christmas) and runs through January 6 (the Epiphany, sometimes also called Three Kings' Day). The four weeks preceding Christmas are collectively known as Advent, which begins four Sundays before Christmas and ends on December 24."

https://www.vox.com/21796404/12-days-of-christmas-explained