American Dad! Season 3 Episode 9

When an injunction prevents Langley Falls from lighting their annual Christmas tree, Stan blames secular non-believers for ruining his favorite holiday. With Stan’s holiday spirit at an all-time low, the Ghost of Christmas Past visits him and tries to show him the true meaning of Christmas. However, Stan is convinced that Christmas can be saved by changing the outcome of Vietnam, but finds that changing the past only dramatically changes the future.