Summer: the right time for anything structural
Masonry repair, crown casting, rebuilding and flashing all need a curing window above freezing, and they need a settled stretch of weather to be done well. Summer is also when contractors have the capacity to do a proper job, not fitting it between urgent calls. If an inspection last winter identified structural work, this is the season it belongs in.
Late summer into early fall: service and sweeping
The window for sweeping, inspection and appliance service before first firing. It leaves time to order parts, and it means a fault is found before you need the appliance instead of during the first cold snap. Gas appliances belong here too: the annual service is what prevents the pilot that will not light on the first cold evening.
Winter: what is still possible
Sweeping and inspection continue, though roof access is weather-dependent and the appliance has to be cold, which means a day without heat. Mid-season cleaning on a hard-burning stove is a normal winter request. What cannot happen correctly is mortar work, so a chimney that fails in January commonly gets stabilised and weather-protected until spring. Plan for it this year.
Spring: the underrated slot
Booking a sweep and inspection in spring has two real advantages. Deposit is not sitting in the flue through a humid summer producing the odour complaints, and any structural finding lands at the start of the masonry season, not the end of it. The least contested part of the calendar.
Household checks between service visits
After winter, photograph changes in the stack from the ground: loose brick, a damaged cap or new crown cracks. In summer, prepare the fuel supply and keep pellets dry. Before fall firing, test smoke and carbon monoxide alarms according to their instructions. During winter, keep exterior vent terminations clear of snow and ice and report changed draft or appliance behavior.