The Romans called the hottest days of summer dies canicularis, Latin for "dog days." They associated the heat with the "dog star" Sirius, the brightest star in the night sky which happens to be found in the Canis Major (big dog) constellation. They even sacrificed a dog each year at the beginning of the dog days to try and appease Sirius.
The Greeks were in on the dog days too, as were the ancient Egyptians who associated the seasonal flooding of the Nile with the appearance of Sirius in the early morning sky just prior to sunrise.
In modern times, folks have associated the term "dog days" with a sluggish stock market, calling poorly performing stocks "dogs." The terms "dog tired" and "dogging it" may have come from the lethargy that many people feel during the hottest, most humid days of the year, even though the originial "dog days" name had nothing to do with an actual dog!
So, another 90+ day today, and it's expected to be at least that hot on Friday. And temps are staying in the mid-upper 70s at night, which means there is no relief from the heat and humidity.
Things may cool to the upper 80s by Saturday when a storm system will move through the area. Starting Saturday night, the evening temperatures should be in the mid 60s, which should help cool things off.
In the meantime, be sure to leave plenty of water out for the dogs to drink - it'll help them get through the dog days!