Personally, I'd say early game turns to mid game around fall of year one, (when, in my experience, my economy starts to kick off relative to what it had been up until then, such that time becomes a more important limiting factor than energy, and when the greenhouse is (usually) unlocked which opens a lot of options) and mid game turns to late game when the golden walnut room is opened.
That said, I also do a lot of short runs too, which can skew perspective. Like my current run, early game was getting Harvey to four hearts so I could dance with him at the spring festival, mid-game will be ramping up to marriage, and late game grinding out those last 14 hearts (and second house upgrade) to see his 14-heart event.