In yaoi fanfiction, Zak Monday and Zak Saturday are likely names given to characters. Yaoi often delves into the complex emotions and relationships between male characters. Maybe Zak Monday is more of an outgoing character and Zak Saturday is a bit more reserved. Their stories could involve themes such as first love, misunderstandings, and ultimately, a deepening of their relationship, but without more context, it's hard to say exactly what their roles are in the fanfiction.
I'm not entirely sure specifically who Zak Monday and Zak Saturday are in the context of yaoi fanfiction. It could be characters created by a particular author or within a specific fan community. They might have unique personalities and relationships that are explored in the yaoi stories.