Question: "'Tis the Fifteenth Season" is the seventh episode of "The Simpsons"' fifteenth season, and the seventh Christmas-themed episode overall, this episode is notable as being the first to make reference to Lisa's Buddhism since she converted in which earlier Christmas episode, which is the sixth episode, of "The Simpsons"' thirteenth season?
Sentence (from "'Tis the Fifteenth Season"): It first aired on the Fox network in the United States on December 14, 2003.
Relevant evidence: No