"Oh, to be in England now that April's there", quoted Austen contemporary, Robert Browning. No doubt he was referring to the lovely countryside, abloom with spring's bounty (Wordsworth's Ddaffodils, perhaps?) rather than the hustle and bustle of a large city like London, where the social season was still in full swing.
The London social season evolved in the 17th and 18th centuries, and in its traditional form it peaked in the 19th century. In this era the British elite was dominated by landowning aristocratic and gentry families who generally regarded their country house as their main home, but spent several months of the year in the capital to socialise and to engage in politics. The most exclusive events were held at the town mansions of leading members of the aristocracy. Exclusive public venues such as Almack's played a secondary role. The Season coincided with the sitting of Parliament and began some time after Christmas and ran until midsummer, roughly late June. The social season also played a role in the political life of the country: the members of the two Houses of Parliament were almost all participants in the season. But the Season also provided an opportunity for the children of marriageable age of the nobility and gentry to be launched into society. Women were formally introduced into society by presentation to the monarch at Court.*
Wealthier young ladies, mad to enjoy each hour of their limited time in the cosmopolitan capitol, would scour fashion magazines, like Ackermann's Repository, to see the latest fashions and ensure that their gowns (sometimes worn only once or twice before being replaced by newer dresses) were of the latest mode. Less affluent young women, like the Austens, might use the plates as a guide for re-trimming their gowns time and time again. Sometimes adding a flounce, or new ribbon was just what was needed.
This fashion plate from Ackermann's Repository from April, 1812, offers a glimpse of a stunning ball gown, in an exotic style. At her website, Jane Austen's World, Vic Sanborn offers an in depth look at this dress and the fashion aspects that went into creating it. One can just imagine Caroline Bingley attempting to carry it off with aplomb. There are many costume patterns and accessories available from the Jane Austen Giftshop
*Historical information from Wikipedia.com