The Question and Answer section for All Souls: A Family Story From Southie is a great resource to ask questions, find answers, and discuss the novel.
Michael's brother Davey was under the care of a doctor at Mass Mental. The doctor would go on vacation for two weeks each August, and when he did, Davey would always suffer a breakdown. (Page 128)
GradeSaver doesn't have a summary for this but try the link below, it may help your memory.