Looking on the internet deeply has found these results:

at the peak of summer. is the most popular phrase on the web.

at the peak in summer. 11 results on the web	at the peak of summer. 1417,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
towards the end of 2009. I'm at the peak of my physical beauty. Lily was right at the peak of her career... The communication from the Commission was drawn up in the wake of the alarming oil price peak early last summer. And I am at the peak of my manhood. Energy, industrial and food commodity prices plummeted from their peak in summer 2008 to a trough in winter 2008 09, to rebound again in most cases	...that existed at the peak of the imperial Roman Empire. I'm at the peak of my physical beauty. The communication from the Commission was drawn up in the wake of the alarming oil price peak early last summer. You're at the peak of your powers. If it happened at the peak of the spring tide for maximum effect... This is Terra Dino at the peak of the Cretaceous period. Lily was right at the peak of her career... And I am at the peak of my manhood.

at the peak in summer.

11 results on the web

at the peak of summer.

1417,000 results on the web

More popular!

towards the end of 2009.
I'm at the peak of my physical beauty.
Lily was right at the peak of her career...
The communication from the Commission was drawn up in the wake of the alarming oil price peak early last summer.
And I am at the peak of my manhood.
Energy, industrial and food commodity prices plummeted from their peak in summer 2008 to a trough in winter 2008
09, to rebound again
in most cases

...that existed at the peak of the imperial Roman Empire.
I'm at the peak of my physical beauty.
The communication from the Commission was drawn up in the wake of the alarming oil price peak early last summer.
You're at the peak of your powers.
If it happened at the peak of the spring tide for maximum effect...
This is Terra Dino at the peak of the Cretaceous period.
Lily was right at the peak of her career...
And I am at the peak of my manhood.