"All in a day's work" basically refers to the usual tasks that someone has to do in their job but may be unfamiliar or unusual for other people. For example "The policeman arrested another criminal yesterday. It was all in a day's work for him." This indicates that it's usual for the policeman to arrest people in his work but probably not normal for most people!

"All in a day's work" basically refers to the usual tasks that someone has to do in their job but may be unfamiliar or unusual for other people. For example "The policeman arrested another criminal yesterday. It was all in a day's work for him." This indicates that it's usual for the policeman to arrest people in his work but probably not normal for most people!