The 12 Days of Christmas start on December 25 (Christmas Day) and continue through January 5, ending in the evening of January 5, which is called Twelfth Night. The next day, January 6, is Epiphany or Three Kings' Day, marking the conclusion of the Christmas season. These 12 days are a traditional Christian festive period celebrating the birth of Jesus and various associated feast days, with December 25 as Day 1 and January 5 as Day 12.

During these days, many Christian denominations observe different celebrations and saints’ feast days, with Christmastide lasting from Christmas Day to Epiphany Eve (January 5). This period has been celebrated since the Middle Ages and was officially recognized at the Council of Tours in 567.

In summary, the Twelve Days of Christmas are from December 25 to January 5, culminating in Twelfth Night on January 5, with Epiphany on January 6 marking the end.