Examples of early Nineteenth Century hat trimmings, such as the one Elizabeth might have been working on:

Assemblies refers to social events that were held in local Assembly Rooms for the public. They were often balls, as in this case. The Assembly Rooms were large public spaces built for such an occasion.

According to the Oxford English Dictionary, “hack” means “a horse used for hire” (OED), and “chaise” means “a light open carriage for one or two persons” (OED), so Mrs. Long arrived in a hired carriage.

An old spelling of "choose"; To take by preference out of all that are available; to select; to take as that which one prefers, or in accordance with one's free will and preference (OED).

Fortnight: A period of fourteen nights; two weeks (OED). Therefore, "To-morrow fortnight" would be two weeks from tomorrow.