I agree, it's absolutely about getting to know someone new, but I think it's more about the transition from a just dating someone to establishing a relationship. "September's still summer but the nights are like fall" is a reference to spending the whole night talking with someone and getting to know them better. The changing of the seasons also alludes to the growth of the relationship from the heat and casual fun of summer to the comfort and relaxation of fall.
I agree, it's absolutely about getting to know someone new, but I think it's more about the transition from a just dating someone to establishing a relationship. "September's still summer but the nights are like fall" is a reference to spending the whole night talking with someone and getting to know them better. The changing of the seasons also alludes to the growth of the relationship from the heat and casual fun of summer to the comfort and relaxation of fall.