I've noticed the seasons change sporadically on my android phone. I started in Fall, which lasted about a week. Winter lasted maybe 3 or 4 days, Spring only 1 or 2 days, then Summer for almost a week. I play almost every day, so I'm not sure what triggers the season changes.
OP: Sounds like you're in Summer. Try playing a bit more and/or leaving the app open for a little bit each day and maybe it'll trigger Fall to come.

Spinach and Salad Leaves have the same icon, but they are not interchangeable in recipes. Salad Leaves can be harvested from any field in the Summer season.