I guess what's bothering me about this is the "No reason given for the dinner" bit. It smacks of "Just make time in your personal life for something you know nothing about just because we say so." I find it more than a bit rude to make that kind of assumption on your personal time.
It reminds me of a professor in college who was annoyed that I wasn't going to attend some "optional" outing to a local museum, because my father was having a quad bypass that day and I was going to be at the hospital from 6AM until whenever. Frankly, even if it wasn't "optional", I wouldn't be attending. I was going to the hospital - period. I know your situation is different, but the attitude is the same.
Maybe it is some kind of nice surprise, but if you're going to do that, do a luncheon or reception during working hours.
Just my opinion. (Which is worth exactly what you paid for it.
)
