The dog days of summer are the hottest days in summer:
During the dog days of summer he loved to spend the afternoons down by the river.
Other expressions related to the summer and sunshine include:
ray of sunshine = a person who is always happy and pleasant
My younger brother is a ray of sunshine. He always cheers me up when I feel fed up.
walk on sunshine = feel very happy
She’s been walking on sunshine since she passed her driving test.
Make hay while the sun shines = to make the most of an opportunity
As the cost of raw materials has gone down, the company has decided to increase its production and make hay while the sun shines.
To take a shine to someone or something = to develop a liking for
After his trip to the USA, he has taken a shine to driving automatic cars.