"Brother Day of the Week on Sunday" centers on the life of Haruto, a high school student who discovers he has seven brothers, each named after a day of the week and representing distinct personalities and traits. The brothers, who were separated at a young age due to familial circumstances, reunite unexpectedly, leading to a complex dynamic as they navigate their shared past and present relationships. Each brother embodies a specific day’s characteristics, influencing their behavior and interactions. Sunday, the youngest and most carefree, serves as the emotional anchor of the group, while others like Monday, the disciplined and responsible eldest, and Friday, the rebellious and enigmatic middle child, bring tension and balance to the family dynamic.

The narrative explores themes of identity, belonging, and reconciliation as the brothers grapple with their fragmented upbringing and the reasons behind their separation. Haruto, initially overwhelmed by the sudden influx of siblings, gradually forms bonds with each brother, uncovering their individual struggles and aspirations. The story delves into their efforts to rebuild trust and understanding, often juxtaposing moments of humor and heartbreak. Subplots involve the brothers’ interactions with external characters, including classmates, love interests, and figures from their past, adding layers to the central family drama. The manga balances slice-of-life elements with emotional depth, portraying the brothers’ journey toward unity and self-discovery.