The Question:

Could you find me information about the old myth about the "Dog Days of Summer?"

The Answer:

The origins of the saying The Dog Days of Summer comes from early times. According to 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.

—The Fact Monster