The viable season is May or June through mid to late October.
a. Some of the roads are closed during the winter due to snow. This rules out November through April. The snow is cleared from the highest roads sometime during May or early June, depending on the snow pack. Early October is generally OK, late October or early November might be OK too – it all depends on when the first storm dumps snow.
b. Northern California has a wet and a dry season, and you certainly want to ride this route during the dry season, as the wet season in Northern California can be REALLY wet; for instance, Honeydew averages 80 inches of rain per year, nearly all during November through April. On the other hand, the dry season is usually completely dry and you are likely to have a trip with no rain. The wet season is typically November to March. October, April, and May are transitional months that may or may not have much or any rain. Fortunately, this aligns with the snow closures.
c. The grass is green and the flowers are in full bloom during April and May. Things start to dry out as soon as the rain stops. By the end of summer, everything is dry and golden. It is often less windy in the fall than spring and summer.
d. The coast and low-lying valleys are nearly always foggy during July and August. Sometimes that fog will burn off by mid day, but not always. If you travel during July or August you might experience dense fog while near the ocean. On the other hand, the view from the hills and mountains when the fog is below you is delightful. September and October have much less chance for dense prolonged fog.
e. Much of this route can be quite hot during the summer. It can be too hot to enjoy being out in the sun. If you are used to temps in the 90s or more, then don’t worry about it, but if you are a fair weather rider, then avoid July and August (and even June, as Andreas will attest).
f. The road through Lassen National Park reaches 8500 feet, and that road is closed from the first serious snow (usually in late October) until the road is cleared of snow, usually in May or June. Fall closure is not predictable, in 2007 the road closed unusually early on October 7th. In 2008 it did not close until November 11th. It’s almost certainly going to be closed by mid November of any year. On the other hand, the opening date of the Lassen road may be fairly predictable by March or April. You can look at the snow pack report in the spring - look at the top plot on this page (Northern Section Trinity River through Truckee). http://cdec.water.ca.gov/cgi-progs/snow/PLOT_SWC
If you call Lassen National Park in April they might be able to give you a forecast for an approximate date the road will open, based on the work schedule.
g. Big Leaf Maples, Black Oaks, and Dogwoods put on a nice autumn show; depending on the location, they peak sometime in October.
h. We took the trip from October 5 to 18, and had clear, cool, calm, pleasant weather every day. Our only windy stretch was between Petrolia and Eureka; the exposed coast there is (we believe) windy all year, and the wind is consistently out of the North or Northwest. We had temperatures near or below freezing on half of our early mornings, but the daytime temperatures were in the 60’s.
i. If you are a salmon or steelhead fisherman, you might want take that into account when choosing your dates.