The Dog Days is the time of the year when the Dog Star, Sirius rises with the sun. They run from the 3rd of July to the 11th of August and were known from Roman times as the hottest days of the Summer. The Romans called these caniculares dies, and it was reasoned that since Sirius was so bright, when it rose during the day time it added its heat to that of the sun.
This extra heat plus the influence of the Dog Star was said to make humans and animals behave oddly or out of character, making dogs more likely to bite and humans to be subject to fits of temporary insanity, or to be more likely to commit acts of violence. Although the ancient Greeks believed that the Dog Days made men more tranquil and women more volatile.
The Dog Days were also believed to give a general indication of the health or wealth of the nation:
Dog Days bright and clear
Indicate a happy year:
But when accompanied by rain
For better times our hopes are vain.
The ancient Egyptians knew the Dog Star well and to them it was the star of Isis. Osiris was linked to the constellation of Orion. Sirius was called Sopdet and was the precurser to the annual inundation of the Nile, which brought fertility and wealth to Egypt.
In ancient Persia, Sirius is Tishtrya and revered as a divinity who brings rain and fertility, forever fighting Apaoasha the demon of drought. Tishtrya is depicted as a white horse.
Sirius is the brightest star in the night sky, and seems to twinkle in different colours. This may be because Sirius is actually a Binary star, two stars which circle each other around a common centre - it has recently been theorised that Sirius might be a system of three stars, the third member being either a small, dead, burnt out star or even a black hole.