If you are in a time crunch but still want something new to wear for the Georgian picnic, I can highly recommend Sensibility.com's Elegant Lady's Closet pattern, specifically the drawstring gown. I've made it five times now, and one of those times I made it in a week, with a newborn baby.
The thing I love the most about this pattern (aside from its sheer simplicity) is its very forgiving design. The drawstring aspect to it allows for a lot of fluctuation in weight of the wearer, so precise fitting isn't really necessasy. This trait also makes it ideal as a loaner when you want to entice friends along on a costumed adventure.
Additionally, if you wear a very "lifting" bra or good fitting sports bra, you can get a decent facsimile of the correct silhouette for the Regency period. I've totally cheated in this way, especially when I still had my little nursling. Ideally, of course, I prefer Regency short stays to get the perfect silhouette. However, if you're in a rush or want to do a quick one-off, then go for it. I also have worn this dress with a long, modern slip, and another time with a Victorian petticoat that I just hiked up to my under-bust with safety-pinned shoulder straps to hold it in position.
As for the pattern itself, the directions are clear and easy to follow, with useful illustrations. I’m a very visual learner, so the illustrations were key. Also, because her patterns are so often used, there are a lot of very useful resources on her website, like tutorials, videos, and other extensive tips and tricks.
Here are the samples that I myself have made…
First run through was in a light weight pink cotton:
Second was the one I put together super quickly, soon after my son was born. This one has an elastic neckline, which was done for the ease of nursing my little one:
And a full length shot:
Third was as a Christmas gift for my niece:
An action shot:
On the fourth go around, I took liberties and turned it into a chemise dress out of a very fine cotton voile. For this one, I simply added extra fabric to the front (bodice and skirt) and extra fabric to the skirt in the back to get the look I wanted.
And the fifth and most recent rendition, I followed a tutorial done by Mme. du Jards Atelier and made a spencer.
I've only made the dress with the long sleeves and the elbow length sleeves, but it also comes with a pattern for short puffy sleeves for another variation.
I've definitely gotten great use from this pattern, and highly recommend it to the beginning costumer for a great start in the Regency era. I also would recommend it to the more experienced costumer who is looking for a quick, simple gown in an era that they may not have done.