Harmful alcohol consumption can significantly compromise adherence to antiretroviral therapy (ART). Prior research has identified aggregate relationships between alcohol use and ART non-adherence, largely relying on concurrent assessment of these domains. There is relatively limited evidence on more nuanced day-level associations between alcohol use and ART non-adherence, despite potentially important clinical implications. We recruited adults with HIV treatment adherence challenges and harmful alcohol use (n=53) from HIV care in South Africa. We examined relationships between alcohol use and same and next day ART adherence, accounting for the role of weekends/holidays and participant demographics, including gender. Results demonstrated that ART adherence was significantly worse on weekend/holiday days. Next day adherence was significantly worse in the context of weekend alcohol use and among men. These results suggest the importance of tailoring intervention strategies to support ART adherence during weekend drinking and for men engaged in heavy episodic drinking.