Originally 'dog days' was all about Sirius the 'dog star' and how it rises and sets with the sun. The 'dog days' dates were between July 3 and August 11. It was a 20 day period that happened to coincide with intense heat.
Because Sirius was the brightest star, ancient Romans believed that the sun heated the 'dog star' Sirius up during the day, therefore causing it to shine so brightly during the night, and the 'dog star' attributed to the intense heat of the period of July and August.
Obviously now, while we use the expression 'the dog days of summer' we attribute it to intense heat, sultry weather and periods of being lazy and inactive (which we blame on the intense heat and sultry weather!)
Obviously now we use the phrase to encompass many days past August 11th. We are still counting lazy, hazy summer days clear into September.
As an English teacher, I love the richness, variety, and complexity of our language. I worry that we skip right past the old adages, cliche's and idioms without every taking the time to know from where they are derived.
My students are usually clueless to what the old phrases really mean. My American born students will tell me that they remember their 'old grandmas and grandpas' saying things like that, and my foreign born students have no idea at all what the phrases could mean.
I put together little powerpoints (similar to what I have just done) and show them on the Smartboard (a big interactive screen). That makes the students seem to be more in-tune and actually 'get it'. Then I feel better that they have at least been exposed to our rich language heritage.
Maybe tomorrow I should explain the origination of "cat naps' here on the blog!!