The London Season began when titled lords came to London to attend Session of the House of Lords which coincided with Parliament. They typically met in October, November, December and then again in January through about April or so. There was really no set schedule, as far as I can see although the Queen held her birthday ball in January.Often the gentlemen came alone, but many probably preferred to bring their families, who then needed entertainment, and thus the Season came into being.
Since London was an undesirable place to live year-round, many would not come to London from their country houses until after Easter when the weather was better. The London Season generally ran from after Easter to June or July, after which most would return to their country estates. Some people lived in London all year round, except for brief visits to other houses. Later, under Queen Victoria, the Season became more definite and the whole debutante thing was formalized.
During the Regency Era, the Season usually included lavish balls, parties, dinners, and musicales. London also offered many other attractions; the zoo, many parks, museums, shopping, and other entertainment. The Season grew into an important part of meeting and marrying eligible gentlemen and ladies.
The season became a much bigger production after the Regency Era and really came into prominence in the late Nineteenth century, or the Victorian days. Still, the Season figures prominently in many Regency Historical Romance novels, including mine, since it was a time to bring together all the noble families and their children of marriageable age. This of course, invites all kinds of delicious possibilities.