Month by month, not "summer is best"
May to October is the short version. Below is what changes month to month inside that window, and what's lost outside it.
May & June: the walkers' favourite, for real reasons
Long daylight hours (sunrise before 5am by mid-June this far north), reliably dry underfoot conditions, and accommodation fully open, but before the late-July school-holiday crowds arrive. Northumberland's exposed uplands can still turn cold and windy even in a nominally warm month — the trail's highest point, Windshields Crags at 345 m, has no tree cover at all.
Late July & August: busiest, and the one stretch that shows it
English school summer holidays concentrate visitor numbers at Housesteads and Steel Rigg specifically — the most photogenic, most road-accessible section of the crags. Car parks at Once Brewed and Steel Rigg fill early on good-weather days. Walking the trail itself is less affected than day-tripping to these two specific spots, since most thru-hikers are moving rather than parking.
September & early October: a second window
Ground conditions are usually still reasonably dry, accommodation is still open, and visitor numbers drop sharply after the school-holiday crowd leaves. This is also broadly when the Sycamore Gap tree's stump — alive and resprouting since the tree was illegally felled in September 2023 — puts on its most visible seasonal growth, if that site matters to your visit.
Winter (November–April): possible, but a different trip entirely
National Trails' own guidance is explicit about why this isn't just a comfort issue: wet ground causes real damage to the archaeology under the path, beyond just a harder walk for you. Add to that a reduced weekend-only bus timetable, seasonal accommodation closures in the smaller villages, and the trail's passport-stamping scheme not running everywhere, and a winter thru-hike needs considerably more of its own research than picking a warmer month does. A winter day visit to a fort with its own year-round opening hours (Segedunum and the larger sites tend to stay open) is a much simpler proposition than a winter thru-hike.
Northumberland's dark skies, if stargazing matters to your trip
The trail's central section runs through the Northumberland International Dark Sky Park, one of the largest protected areas of night sky in Europe — measurably darker, and worth timing around a new-moon date, in the winter months when nights are longest, even though that's the least convenient season for the walking itself. See stargazing along the wall for how to plan around it.