“The season” is notionally when Parliament is in session, from a little after Christmas until August 12. More importantly, however, it is when the most important social events occur. Wealthy people who usually live out of town often come into London for the season. The Debutante system of America is not dissimilar: young ladies must be “presented” in the season, at balls, then taken to dances, parties, restaurants, theatres, concerts, and the like. In theory at least, a young lady who does not make a match after three seasons is deemed a failure; in reality, it’s rather more complicated than that.
Those without young protegées may or may not come down for the season, depending on preference. It is a common trope that ladies like to go, and their husbands do not; alternatively, it is sometimes the older men who like to come down in order to meet pretty young things, not always for immoral purposes. Assuming nothing untoward, older people sometimes like the gaiety and innocence of young society, and as they are likely to be well-connected can be invaluable to a young person without good contacts.
Historical
-- ChrisLehrich? - 24 Dec 2004