(not a teacher)
I'm a native speaker, sorry that I can't shed any more light on the issue though!
It occurs to me that every other general timeframe does not require 'the':
I will be home next week/next fortnight/next month/next year etc.
I believe it has something to do with the fact that you can just say 'tomorrow'. There is no such word for week, month, year etc. I can't quite put my finger on it, so what I'm trying to express won't be very clear!
This is the only thing close to an answer I can come up with; 'the next day' already has a word - tomorrow. Week/month etc don't. So perhaps it is something to do with language change over time - the week/month etc lost their article because it isn't needed. 'Day' kept it's article because it is used far less than week/month etc in the form 'the next ___'.
This is a complete guess!