Today is the summer solstice, the first day of summer and the longest day of the year in the Northern Hemisphere. The changing seasons are a result of the Earth's tilt on its axis; as our planet makes its annual trip around the sun, our Hemisphere tilts toward the sun in the months surrounding June 21 and away from the sun in the months surrounding December 21 (of course, the opposite is true for the Southern Hemisphere). On this solstice, we are at our maximum tilt toward the sun and, from now until December 21, our days will gradually shorten.
While this is the longest day of the year, our warmest months lie ahead. This is due to the continued, gradual warming of the ground, lakes and ocean waters of the Northern Hemisphere, moderation of the Arctic conditions via the perpetual daylight of summer and a "high riding" jet stream, which blocks cool Canadian air from moving southward. We can't expect much relief from the hot, hazy days of summer until this pattern breaks, generally about mid September in the American Midwest.