why did tom feel miserable on the holidays?

Holidays were Saturdays. Tom always felt miserable because he ended up having to work off his misdeeds. 

He'll play hookey this evening, and I'll just be obleeged to make him work, tomorrow, to punish him. It's mighty hard to make him work Saturdays, when all the boys is having holiday, but he hates work more than he hates anything else, and I've got to do some of my duty by him, or I'll be the ruination of the child."