The best times are at the end of the burning season or before the start of the heating season.
End of season benefits - removes all deposits while they’re still manageable and prepares your chimney for the next heating season.
It also prevents acidic soot and creosote from corroding your flue liner over the summer, eliminates smelly, damp odours from sitting in your home, and allows time for any necessary repairs to be done before the next winter, all while avoiding the rush.
Start of season benefits - ensures it is clean, safe, and free from blockages like bird nests after months of inactivity. This timing prevents chimney fires, eliminates hazardous creosote buildup, improves efficiency, and reduces dangerous smoke/odours when you first light your fire.