The Bead Head Mylar Prince Nymph pattern can be fished all season and in all bodies of water. The Mylar Prince Nymph is a variation of the originally tied Prince Nymph, or Brown Forked Tail Nymph, which as we know has morphed considerably in the multiple variations of the Prince Nymphs' pattern.
In streams or rivers, this nymph pattern can be presented in a number of ways. The most successful being a cast upstream like a dry fly, and then allowing it to dead drift drag free, keeping in contact with your line.
Alternatively, allow the nymph to sink slowly to the bottom, just above the vegetation it often seeks and slowly raise the line on the drift, through the water table to attract attention. A few other great ways to fish them are under a big dry fly or hopper dropper rig and a euro nymphing rig.