When to go
There isn't a single "best" month — it depends what you're actually here for. This page covers the real seasonal trade-offs rather than a generic "spring and autumn are lovely" line.
Heather season
The purple heather bloom that photographs so well across the Dark Peak's moorland and parts of the higher Dales typically peaks in a window from mid-August into early September, driven by ling heather (Calluna vulgaris) flowering later in the season than bell heather. Timing shifts year to year with weather, but the second half of August is the most reliable bet if this is the specific reason for the trip — arriving in June or July for "heather season" is a common and avoidable mistake.
Winter and high-ground closures
Neither park closes for winter, but high, exposed routes change character completely. Kinder Scout's plateau and the higher ground on the Yorkshire Three Peaks route regularly see snow, ice, and genuine whiteout conditions in cloud between December and March, and walkers without winter hillwalking experience (ice axe, crampons, navigation in poor visibility) can find themselves seriously out of their depth on ground that's straightforward in summer. Rural roads in both parks, particularly higher unclassified lanes in the Dales, are sometimes closed by snow for short periods — this is a real possibility to check for, not a remote edge case, if visiting between December and February.
Shoulder seasons
Late April to early June, and September into early October, tend to combine lower visitor numbers with generally settled (though never guaranteed) weather, and avoid both the heaviest summer crowds at Dovedale/Malham/Stanage and winter's high-ground hazards. Spring brings newborn lambs across the Dales' pastures — genuinely part of the seasonal character of the landscape here, not just an aesthetic detail — while some seasonal attractions (certain show cave opening hours, some National Trust cafés) run reduced hours outside the main April–October season, worth checking directly before relying on them.
Wettest months
Both parks sit on the wetter, western-facing side of the Pennines relative to much of lowland England, and autumn through winter (October–January) is statistically the wettest stretch — worth factoring into footwear and clothing choices regardless of which month you're travelling, since even a "dry" summer week here can still deliver a genuinely wet day.