Late November through December: the big one
This is the cleaning that matters most. Valley oaks and street trees finish dropping by early-to-mid December, and the serious rain usually arrives right behind it. Book after the bulk of leaf drop, before the first big atmospheric river if you can manage it. If you only clean once a year, this is the once.
September or October: the pre-storm check
A quick look before the first real storm of the season. Summer leaves surprisingly little debris, but one early clog meeting one October downpour is how fascia boards rot. Under heavy canopy this check often turns into a light cleaning. Under open sky it takes ten minutes and costs nothing.
March or April: the spring once-over
Oak tassels, elm seeds, and blossom drop land in spring, and they mat into a surprisingly effective sponge. Homes under oaks usually earn a second cleaning here. Homes without overhanging trees can usually skip it, and anyone who tells you otherwise is selling.
Summer: leave them alone
June through August, dry gutters with light debris are fine. This is the season for gutter repairs, guard installs, and repainting if you need them, precisely because nothing is urgent. A company pushing an emergency summer cleaning in Sacramento is pushing.