Between 1979 and 2011, the absolute sunniest day of the year has been July 8, according to the CDC's numbers, which ultimately come from NASA. On the flip side, the least-sunny day of the year is Dec. 26.
Sunlight here is measured via incoming solar radiation, or insolation (anyone else having flashbacks to earth science class?). Insolation is affected by some obvious things, like latitude (incoming sunlight gets weaker as you move farther away from the equator) and time of year (shorter days equal less sun). But other things affect it, too, such as cloud cover and air pollution.
You can see how this all plays out by looking at a map of average daily sunlight amounts in the U.S. The map below is interactive, so you can mouse over your own county to see how it measures up with everyone else (apologies to Alaska and Hawaii: You guys aren't included in NASA's data).
The sunniest place in the lower 48 is Imperial County, Calif. All that sunlight makes the region a great place to grow crops — or it least it was, when water from the Colorado river was more plentiful. Overall, the Southwest gets the most sun in an average day, along with select regions on the Gulf Coast and the southern tip of Florida.
Average sunlight generally diminishes as you head farther north, though you can see that sunlight isn't simply a function of map placement. Pittsburgh (Allegheny County, Pa.) is at roughly the same latitude as Salt Lake City (Salt Lake County, Utah), but the former is at the bottom of the solar distribution while the latter is closer to the top.
You can also see the effect of landscape features, particularly in the Pacific Northwest. Counties to the west of the Cascade mountains get relatively little sunlight, while counties to the east get considerably more.
The absolute least sunny county in the lower U.S. is Island County, Wash. On an average day, it gets only about 60 percent of the solar radiation of a typical county in Southern California. The states bordering the Great Lakes — from Minnesota to New York — are all at the bottom of the sunlight distribution. Northern New England is right there with them.
But these average numbers don't capture the variation in sunlight an area receives during a given year. To show that, I've made a series of maps showing average monthly sunlight levels in the Lower 48, below (note that the color scale here is different than the one in the interactive map above to capture the wider range of solar radiation values.)
So, if you're like me, stuck in the fetid swamp that is D.C. in July and wishing for cooler days, take heart. Each coming day will be less sunny than the previous — statistically speaking, at least.