The dog days of summer start 20 days before Sirius and Sun achieve conjunction, and ends 20 days after conjunction. It may have been once believed that Sirius and the Sun being in the sky together was possibly the reason it was so hot during the Northern Hemisphere summer, which is obviously nonsense. Regardless, the name still sticks and the dog days of summer are centered on Sirius – the dog star being in conjunction with the Sun.
Image Credit: Visit Rhode Island