The social “Season” is generally described as beginning in early spring and lasting until the end of June. The season had some relation to the sitting of Parliament. It convened each January, so those involved in the government would head back to town at that time. No doubt their ladies spent the next couple of months updating their wardrobes and planning their social calendars for the spring. As for the term “Little Season”, supposedly in the fall — I have never seen any mention of a Little Season anywhere in a primary source. It is only referenced in books by Georgette Heyer and other writers of fiction. It makes sense that there might have been such a thing, as the upper classes who had left London for the seaside or the country might have returned to town in the fall, especially those involved in Parliament, which was still in session until November. But I have never come across the term Little Season anywhere outside of novels.