Thank you all, for the thoughtful answers and suggestions.

The event is Saxon and military art-history in theme. No doubt the day crowd will be 6-10X larger than the evening one.

Still, this is the USA and 21st century, so for evening, some few will be in business suits, a slightly larger number in blazers...the vast majority...both sides of "proper" to some few dressed as though they were doing home plumbing and stopped at the event enroute to the hardware store.

As this writer has engineer mentality and Teuton heritage, a certain exacting part of the brain says things of the ilk:
* At 1800 sharp, sporrans must be bright cantled or full fur mask
* At 1800 sharp, flashes may not be textured fabric with solid colour hose; only castellated hose and tied garters

You get the idea.

"Over-thinking it" (after having read "So You're Going To Wear The Kilt" and so many references here to "day" and "evening" dress has perhaps triggered a bit of paranoia. Yes, "over-thinking it" provides a rather accurate explanation.

Highly unlikely the kilt kops will be there in force with violation books open, tho a number of attendants will be from the isles across the pond. And personal preference is looking smart yet a whiff casual.