British Slang, An expression used to explain or justify a particular incident or series of events because of the existence of an alternative logic applicable to certain social situations.
"Where the fuck has Dave gone? It's his round!"
"He's gone back with that girl"
"Ah fair enough, thats Saturday Logic I guess"
"Sitting in the freezing cold watching a bunch of overpaid footballers lose yet again - and paying for the privilege - now thats Saturday Logic"