The Dog days, a name for the most sultry period of summer, from about July 3rd to August 11th. Named in early times by observers in countries bordering the Mediterranean, the period was reckoned as extending from 20 days before to 20 days after the conjunction of Sirius (the dog star) and the sun. In the latitude of the Mediterranean region, this period coincided with hot days that were plagued with disease and discomfort. The time of conjunction varies with difference in latitude, and because of the precession of the equinoxes it changes gradually over long periods in all latitudes.
Due to a very slow wobble of Earth’s axis, the Dog Star now seems to rise later than it did in ancient times. Its ascension no longer coincides with the start of the Nile flood (which does not occur anyway, because the river is now controlled by the Aswan Dam), but Sirius still makes its appearance during hot summer days.
Old-timers believed that rainfall on the dog days was a bad omen, as foretold in this verse:
Dog Days bright and clear
Indicate a happy year;
But when accompanied by rain,
For better times, our hopes are vain.
Dog Days are approaching; you must, therefore, make both hay and haste while the Sun shines, for when old Sirius takes command of the weather, he is such an unsteady, crazy dog, there is no dependence upon him.
–The Old Farmer’s Almanac, 1817