Technically, the answer is a day dress - which i take to mean something you would wear to the races - i.e not too strappy or short, not to sparkly or nightclubby - just a nice dress - something summery would be appropriate i think. I would suggest that it is a dress more casual then a cocktail wear dress.
Casual business dress. Something you would where to work if you have to look professional, but not dressy. A nice pant suit; a pair of dress pants or a skirt and a blouse; a conservative, (but feminine is OK) dress.