I thought of 3 possible reasons for McCain wanting to cancel:
-lower expectations (see: this entire thread. "omg, he's so weak he needs to cancel," to the point where even a half-decent performance might look stellar compared to the low expectations for him and high expectations for Obama)
-friday night is commonly regarded as where television programming goes to die; with foreign policy being his supposed strong suit, he could be attempting to get the debate on a night where people are actually watching tv
-friday is the sabbath and he doesn't want to force Obama to betray his jewish faith by working on friday night
-lower expectations (see: this entire thread. "omg, he's so weak he needs to cancel," to the point where even a half-decent performance might look stellar compared to the low expectations for him and high expectations for Obama)
-friday night is commonly regarded as where television programming goes to die; with foreign policy being his supposed strong suit, he could be attempting to get the debate on a night where people are actually watching tv
-friday is the sabbath and he doesn't want to force Obama to betray his jewish faith by working on friday night