on the fucking weekend okay

And another thing ... a teacher of X as a foreign language tried to correct my English today. He objected to my saying "on the weekend". "At the weekend" he asserted was the only correct way of encoding this location in time. I nearly fell over. He did not accept my argument that we were both making analogies with other locations in time "on Monday" and "at the end of the world". Then he began a rant about the tragic demise of genitive marking in modern spoken German, which of course made me capitulate weeping for the loss of that lovely innocent case. Mmm. I wouldn't mind such misguided pedantry from a lay person, but from a language teacher? How dare he not recognise competing grammatical systems! How dare he privilege his presumed (hyper)correct form over another native speaker's?
What must his classroom be like? Bored students chanting paradigms of declinable adjectives? God preserve us from language purists.