"In", "of" and "for"

My Murphy book says, when you express about time that takes to do something, you use "in".
For example;

"I learned to drive in four weeks."
"I read this book in a day."

I see. But when I want to say those sentences, I probably say "for" instead of "in" unconsciously. Meanings are changed... right?

The Murphy books teach me one more thing.

"What was the happiest day of your life?"
"Yesterday was the hottest day of the year."

"Of"? It's not "in"? Why??
I still think "in" is more appropriate. Haha.