It doesn't exist.

In a way, it's a joke. I've never been a fan of dress standards. Every time I read one I think "what does that mean, why am I supposed to know what that means?" So we made one up. Although, we did read "Saturday Best" once and adapted it. But it seems "Saturday Best" doesn't exist either. Whoops.

We mean, if we must explain, not as dressed up as "Black Tie", but more dressed up than "Smart Casual".

If you want to dress "Black Tie", that is fine, awesome, excellent. If you have nothing that you think is better than "Smart Casual", don't stay away for fear of offending.

Please relax, we'll be doing enough stressing for all of you.

Update: Apparently it does exist. See Wiktionary "Sunday best".