In answer to the question, "Never."
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Traditionally, Christmas begins on Dec 25th. That is when (if you're a Christian) you should trim the tree and all that. Christmas proceeds until the 12th Night, when the three kings reached the baby Jesus in adoration and presented the gifts (of the Magi). That is when you should open your Christmas presents and celebrate the birth of the Christ Child.

The time (roughly) between Thanksgiving and Christmas is Advent. Modern commercialism has changed the Christmas season to the time period that was traditionally the Advent season, a time of anticipation in which the Christian studies hebraic scripture that foretells the messiah.

Hardly coincidentally, the dates chosen coincide with important pagan calendar events that predated Jesus by over a thousand years, subsumed by Roman Christians and renamed to correspond to their own religious paradigms.