A more gentle pair of parentheses around our human defined "trout season" is really one, not two events. It just happens to start towards the end of our trout season and ends a little while after the beginning of the next trout season. It is the appearance of that lovely winter ephemerid the Large Dark Olive. See that dun drifting down river from September to May? Check the wing position. If it is more vertical than the somewhat swept back wing of the Blue Winged Olive and the fly seems to be just short of half an inch long and its wings are grey, rather than bluish, and, if you can catch one to get a closer look, it has only two and not three tails, then there is a good chance it is a Large Dark Olive.
You can represent a Large Dark Olive very nicely with a variant of the Grey Duster tied on a slightly long shanked size 14 hook. The variation from the original is simply that the fly is dressed with a tail. As follows (just click on a picture to see it in full size):
With the hook point masked by the vice run on a bed of light brown thread from a space behind the eye down to the start of the bend.
Take a bunch of 12 or so hackle fibres from a large badger cock hackle and measure the length to be a little bigger than the body
Swap hands with the fibres and tie them in at the hook bend
Wind the thread along the hook shank tying in the fibres completely and making a bed of thread for winding the body over
Wind the body, if all goes well the dubbed fur will be used up as you reach the end of the body
Tie in a Badger Cock Hackle like this with the concave, dull side towards you. Wind the thread over the stripped portion of the stem until you reach the start of the body. Snip off the hackle stem as close as you can.
Wind the hackle back to meet up with the body, six turns on a fly this size (14). Tie it in, tweak off the waste hackle tip and wind the thread quickly through the wound hackle to avoid trapping too many hackle fibres and make a whip finish. Varnish it and clear the eye of the hook whilst the varnish is still wet. The hackle tip left in your hackle pliers is a perfect pull-through for the job.
This fly is one of the most versatile patterns ever created. Without a tail it makes a great midge. In big sizes with some extra hackle it makes a first class representative of the Drake. Changing the colour of the materials allows you to copy a very wide variety of olives. It is easy to tie and fairly quick so you can soon have your box stocked with a fly that will help you catch fish on most days of the season.
To help folks find suitably marked Badger Capes here is a picture to illustrate the ideal:
The one on the right hand side is wrongly marked. Badger hackles have black lists down the centre of each feather AND they also have black edges to each feather. Click for a closer view.