The Challenged series of flies are excellent emerging mayfly patterns that are very popular and very effective.
As long as the water is above 40 degrees Fahrenheit, the BWO’s will hatch. Look for the major hatches in alkaline waters. Water with vegetation or shallow gravel runs are going to be the most successful for anglers.
For time of day, the best option is midday to evening. These flies will hatch during the warmer parts of the day. Also, these flies are going to be some of the most active during bad weather. Even if there is a storm brewing, these flies will hatch in major numbers so don’t run away from the water too soon.