In chapter 4, we discover why Tom doesn't like Sundays. One reason is that he has to take a bath and wear church clothes. Tom doesn't like baths:
Mary gave him a tin basin of water and a piece of soap, and he went outside the door and set the basin on a little bench there; then he dipped the soap in the water and laid it down; turned up his sleeves; poured out the water on the ground, gently, and then entered the kitchen and began to wipe his face diligently on the towel behind the door.
Not only does he have to be clean, but he also has to wear Sunday clothes--including shoes!
Another reason why he doesn't care for Sundays is that he has to learn his "lesson" before going to Sunday school. His lesson is to memorize a Bible verse, which he has trouble doing. The children earn tickets for each verse they memorize, and because he has such trouble learning the verses, Tom trades things for tickets. That works fine until he's recognized for having 2,000 tickets but can't answer a simple question about the Bible.