The best time to try to hook up with a Marsh Muskie on a fly rod is definitely the springtime, not long after ice out and in the shallows. When the temps in shallows hit the mid 40's to low 50's is usually best.
Emergent weeds and/or wood or other cover in the shallows are good places to target.
Later in the spring and summer, the water warms and Muskies tend to be deeper. Not ideal for fishing with a fly rod.
The whole thing works the same way in reverse in fall. When the temps lower, the muskies will patrol the shallows again.
With muskie fishing you must be willing to sling big flies, be persistent, and not be bothered when you get skunked. A good day is getting a follow or two.
If you stay with it, you should eventually hook up with one and maybe land it.
Every so often, you may get a bonus and catch a larger bass on your muskie rig.
Good luck to all.