Thanks: I was thinking about a lesson for Severus to be giving Harry and, of course, Defense is the obvious choice as Severus loves it and Harry needs it. Quirell is a bit useless in the canon- no point teaching your prey to defend themselves- so I thought it'd be fun to give Severus free choice as to subject-matter: it seemed in character that he'd choose the two, foundational duelling spells plus a charm to ensure that, if your wand goes awry, you can get it back.  

P.S. In almost all English schools, half-term holidays are around Halloween and I thought this would be a good excuse for Severus ^^. Besides, even though we're not told about it in the cannon, this might just be because nothing significant happens during half-term! It's more probable than not that Hogwarts would have half-terms; it would give the students a chance to catch uo on their schoolwork, get to know people from other houses/years and attend remedial classes if they needed them. (Not to mention that the muggleborns would probably riot if there were three whole months of school without a week off in the middle? Evil!)