Shoot at an ISO level that ensures your pictures will be sharp. I know Steve mentioned not needing to go as "high" as ISO 400, but at 7:00 PM-7:20 PM, ISO 400 might be too low of an ISO to ensure you movement-stopping capability; a lot of it depends on the light level, and what kind of gear you have, both camera, and lens(es). If you have a newer, modern d-slr that shoots great up to ISO 6,400 it's "one thing"; if you have an older Rebel or lower-spec'd Nikon that starts to get pretty badly noised-up at ISO 800 and higher, it's "another thing". We don't really know what you have to work with...so...
I have seen (and myself actually screwed up!!!) more situations by staying with too low of an ISO setting to ever think of advising people to limit themselves to ISO 400, or lower, on events that happen during the twilight period. Especially if I think they might well be stuck using f/3.5~5.6 lenses like 18-55 kit zooms, or something similar, which tops out at f/5.6. You mention the golden hour...yeah...maybe there will be sunlight slanting in....or, maybe, it'll turn out to be an overcast day, and you'll be sitting there looking at f/5.6 at 55mm at 1/40 second at ISO 400. It's almost always better to have a slightly noisy image that is basically sharp, than a low-noise, perfectly-exposed series of smeared, blurry frames that are mostly unusable. So, if the situation or your gear, or more likely the combination of situation and your equipment, dictates, jack the ISO to whatever is needed, when the day arrives and you are there.
Hope for the best, prepare for the worst. I remember one July 4 parade where it was raining, and people had old woolen blankets over their shoulders as they sat and stood and watched a windy, cold, rainy parade. 1983. Man, what a sucky July 4 that was. I had prepared to shoot beautiful, blue-sky + white cloud Kodachrome 64 slides, but instead had to switch to grainy, expensive high-speed Ektachrome just to gain some speed.