Generally Autumn and Spring.
Most people believe Autumn fall alone is the best time to clean out the gutters. In a lot of cases, this is correct but It can depend on a number of factors. Obviously, leaves can block gutters, but grass, weeds, and plants can also block gutters quickly and very easily too and they are denser in the gutter preventing rainwater from escaping.
The weight of this debris can be significant for the gutter to cope with which can damage the gutters and fascia boards while the roots damage your gutter joints and seals.
Grass, weeds, and plants flourish in Spring/Summer and moss can block gutters all year round. Get in touch for more help and information.