That Sunday was the dreariest one he had ever experienced. He had experienced several other very dreary Sundays, so for that Sunday to be the dreariest meant it had to be really dreary to be worse than the others.
Looking at the context, Mrs. Cobb is making him feel guilty for making extra work for his mother. He got into a fight and got his clothes bloody and now she has to work to get them clean. As a minister's son, he is expected to be better behaved than other children and not get into fights. He has a mental list of things he's not allowed to do and now he adds one more thing to that list.