Update:
Last Saturday's party was a success! Weather stayed dry, fairly mild, though a bit ominously cloudy and very gusty at times (being lightweight in wind was the dress's only failing. Let's just say I'm glad I bothered with underwear 😳)
We stayed outside for a bit, me wrapped in a shawl, to honour the "garden party" theme, but retreated indoors for the most part.
I enjoyed wearing it, had some nice compliments, and will definitely be getting my money's worth by wearing it at other occasions.
Thanks so much for everybody's help.