Samantha has had plans for this ballgown for several years now. There are lots of sketches of the skirt and the bodice and also an accompanying hairstyle that can be found jotted down in her sketchbook. She saw this design in a magazine called the "Godey's Lady's Book" which was very popular in her time, and she has been desperate to copy it in blue ever since.
Samantha Parkington's Dresses:
So Samantha is always sewing another dress so she can wear it to proper school, or borrowing one of Rebecca's dresses if she isn't done in time.