It was in the original Christmas show---the one that introduced Mr. Hanky. Seems Kyle's mom was at it then too---protesting that the Christmas pageant (that showed Wendy "giving birth" to baby Jesus...) was inappropriate since her Jewish family (among others) did not celebrate Christmas. This opened a HUGE can of worms---soon they discovered that practically EVERYTHING offends SOMEbody!! Mr. Garrison, after informing the kids that a new law was passed that forbad any singing about Jesus or Santa Claus, asked, "Does anyone know any non-Santa or non-Jesus Christmas songs?" to which Cartman responded, "How 'bout we sing "Kyle's Mom's a Stupid B*tch, in D minor?" The rest is South Park history!!