Maybe we're all scratching our heads and hoping someone else will have an actual source, not just a feeling? (Or else it just doesn't seem like that big a deal...)
Yes, 'in the <season>' is the normal way to say it (in BE also, I thought). But I'm not sure I always hear 'in' alone as incorrect.
Thunderstorms often sweep across the plains in summer.
Leaves turn in autumn.
Not wrong, is it? Maybe omitting the article is just slightly more elevated, literary, poetic? (And it's less likely with 'fall' because 'fall' is more colloquial? 'Autumn' BTW is not exclusively BE.)
And/or maybe it's only possible for a general description as opposed to a specific event?
We're going to Europe in the spring.
*We're going to Europe in spring.
Many of the refugees died of exposure in the winter.
*Many of the refugees died of exposure in winter.
That seems plausible, but the more I think about it the less sure I feel. Maybe someone else will have a better idea. In any case, I'm not sure it's something you can cover in a short dictionary entry; it seems more like a grammar topic.
BTW, could I make a similar plea for Muttersprachler front and center? In New/Wrong Entry in several cases I had some questions about possible secondary translations that no one has yet responded to, and I'm not sure whether it's because everything I suggested was okay (surely unlikely), or not okay, or people just didn't read all the way down because the thread title alone looked okay. In particular, 'off the mark' and 'off base' under 'wide off the mark' (WE); and 'bog down,' 'mark time,' 'be quick off the mark,' 'make one's mark,' and 'get full marks' (NE). If anyone has time to cast a critical eye over any of those, even in part, I'd really appreciate it. Thanks. (-: