Q: Context: The Twelve Days of Christmas, also known as Twelvetide, is a festive Christian season celebrating the Nativity of Jesus Christ. In most Western ecclesiastical traditions, "Christmas Day" is considered the "First Day of Christmas" and the Twelve Days are 25 December -- 5 January, inclusive. For many Christian denominations; for example, the Anglican Communion and Lutheran Church, the Twelve Days are identical to Christmastide, but for others, e.g., the Roman Catholic Church, "Christmastide" lasts longer than the Twelve Days of Christmas.

Question: when does the first day of christmas start? Let's give stream of consciousness first:
To answer the question, consider the following: In most Western ecclesiastical traditions, "Christmas Day" is considered the "First Day of Christmas" and the Twelve Days are 25 December -- 5 January, inclusive. Therefore, the final answer is Christmas Day.