May, June, and September, honestly.
July and August have the longest days and the warmest weather, and they're also when the route is most crowded and the midges are worst. The shoulder months trade a little weather risk for noticeably better odds on both fronts.
May and June
This is the strongest window for most first-time drivers: daylight already stretches past 9:30pm by late May, midges haven't built up their peak numbers yet, and the route hasn't hit its July–August traffic volume. Weather is genuinely variable — a warm, still week is entirely possible, so is a cold, wet one — but that variability is true of every month here, not a specific downside of early summer.
July and August
The longest days of the year — near-permanent twilight rather than full darkness this far north around the summer solstice — and the warmest average temperatures. Set against that: peak midge season, UK and European school-holiday traffic that makes single-track passing places genuinely busier, and the tightest accommodation availability of the year in Applecross, Ullapool, and Durness especially. Worth it if you want the long evenings; worth planning accommodation weeks or months ahead if you go.
September
The other strong shoulder-season pick: traffic and midges both drop off noticeably from August, the landscape starts turning toward autumn colour on the moorland, and daylight is still generous enough for a full day's driving and stopping. The stag rut starts building toward the end of the month, adding a genuine wildlife reason to be here rather than just avoiding downsides.
October through April
Genuinely quiet, genuinely cheaper, and genuinely riskier for road conditions — the Bealach na Bà in particular can close in severe winter weather (see the driving notes page), daylight hours shrink hard by December, and several smaller seasonal attractions and B&Bs close outside the main season. Doable, and rewarding if you want the route without the crowds, but check accommodation and attraction opening dates individually rather than assuming July's options are all still running.
The honest recommendation
If you can only pick one window and have no fixed reason to travel in peak summer, late May into June is the strongest all-round choice on this route: long daylight, manageable midges, and accommodation you can still book without months of lead time. September is the close second, particularly if autumn colour and a quieter road matter more to you than the very longest evenings.