Complex sentence: In 2004, many of the stations airing "American Top 10" had begun a programming stunt where instead of mixing in songs from the holiday seasons of the past with their normal playlist, they would switch to an all-holiday format from Thanksgiving to Christmas.
Split into simpler sentences: In 2004, many of the stations airing "American Top 10" had begun a programming stunt during the latter portion of the annual holiday season. Instead of mixing in songs from the holiday seasons of the past with their normal playlist, they would switch to an all-holiday format from Thanksgiving to Christmas.