Question

why would tom sawyer found monday morning miserable

Answer 1

Tom Sawyer found the Monday morning miserable because Monday means the starting of a new week. Which is troublesome for the school children who have to stop their weekend fun and get going to school and back to studies.
This thought of going to school on the Monday morning made him miserable; he wished he fell sick so that he could stop going to school.

Answer 2

Answer:

Tom Sawyer found Monday morning difficult and miserable because it was difficult for him to adopt to go to school suddenly after weekend full of fun