When I have managed to hit one of these hatches in full swing it is an incredible sight, and will rival any black quill hatch for numbers of duns on the water. Blue winged olives (BWO) cover a bunch of different species of mayflies and are a great summer hatch. Because of the different species that all fall under the name of BWO's different hatches happen all summer long and into the fall.
BWO nymphs tend to be swimmers and hatch out to the surface film so the window of opportunity for fish to feed on them is relatively large. I have found that during BWO hatches I have more luck on wets and emergers. So even though there are duns all over the water I usually tend to fish a little Montreal or a pheasant tail, but for those that enjoy hooking fish on a dry here is my favourite fly for BWO's.
And here is a more traditional pattern.