Short answer
In the Treasure Valley, the two best windows are late spring — after pollen and cottonwood fluff have mostly passed — and early fall, after summer dust and sprinkler season but before freezing weather. Winter cleanings work too on dry, above-freezing days.
Late spring: after the fluff
Cleaning in early April feels natural, but in much of the valley pollen and cottonwood fluff keep coming through May and into June, especially near the Boise River. Booking just after that settles down means the clean lasts much longer.
Early fall: before the cold
September and October are ideal. Irrigation is winding down, smoke season is usually over, and the glass gets cleared of a whole summer of dust and spotting before winter. It's also the right time to pair windows with a gutter cleaning once the leaves drop.
What about summer and winter?
Summer is fine for exterior-only refreshes, especially after a dusty stretch or before a gathering. Winter cleanings are possible on dry days above freezing — see can windows be cleaned in winter.
The honest answer: the best time is whenever the glass is bugging you. Regular cleanings matter more than perfect timing, because spots and film are easiest to remove before they build up.
Rather have a pro handle it?
Pro Solutions does window cleaning across Boise and the Treasure Valley. Quotes are free, itemized, and usually texted the same day.