Yep, no matter what time is picked, some people can't make it.
The assumption is that later (9 pm Eastern or thereabouts) would increase participation, and we tried it to see in Battle Over Germany and the proposed Coral Sea. BOG did have large participation, but so did Der Grosse Schlag at 3 pm (which is a very similar scenario). Coral Sea had too low a participation. Also, I have run This Day in WWII on weekends during the day and at night, and I don't see significantly better participation on weekend nights.
My feeling is that Saturday night vs. Saturday day isn't significantly different in participation numbers. There's not enough data to say with much certainty, but if this is correct, it means that at night we get some more Americans but few Europeans, and the numbers are a wash.
Me, I like it best with folks from other countries playing.
The ones who get the worst time are the Aussies. I love it that they play in scenarios, and I do <S> their participation.