Rick ,
It will noticiably darken in a weekend. If its dark wood. Not so much if its light wood. I leave my boats out sometimes over a weekend un varnished to darken the mahogany. I beleive boats darken even with varnish. Its a matter of the beta ray is filtered but not the alpha. Same in sunscreen for your skin. Varnish just slows it down considerably Varnish won't stop a boat from darkening a little, it just keeps your boat from turn all weathered out bleached grey. Bleaching takes a lot of sustained sun to get to. Its not a weekend thing.

Its been my experience that wood that have been sunned before varnishing don't do as much darkening overtime afterwards.

My thought would be the reason you built this was to use it. Rendevous is the perfect place. Go ahead and splash her and hang with some great buds for the weekend. You boat will have its final color by the weekend's end. And thus she will be ready to varnish it when you get home.

Hey winter's coming paddle now

!RUSS