Cytat: Aaric
I stand by my comment;)
Consistency of conviction is a virtue, but let me try to change your mind:
Whenever a particular day is referred to, it should be
the next day. Formality or informality of the language has nothing to do with this.
The article-less
next day is an abstract and time-indefinite floater. Without the article, it is void of referential grounding in time; it is almost like
once upon a time plus one-sided restriction on the time axis.
Compare the two:
One day he had a full head of hair, and next day it was all gone.
The next day would be odd, unless he went bald overnight
Every time I had champagne and brandy before appetizer, the next day I had one hell of a headache.
Next day could mean a medical problem since hangover never returns uninvited.