While the popularity of full-day kindergarten has increased in recent years in the United States, extant research yields inconclusive evidence as to whether full-day kindergarten is more beneficial to children as compared to part-day kindergarten. Previous studies have primarily focused on the quantity, rather than the quality, of instruction. In this study, we examine the quality of instruction and teacher interaction in both full-day and part-day kindergarten classrooms and its relationship to child outcomes at the end of the kindergarten and first grade. Our findings indicate that different aspects of classroom quality may be more salient than the length of the day when examining variance in children’s outcomes. Looking more closely at what actually takes place during the school day will help policymakers and other stakeholders make better-informed decisions when to come to determining policies around children’s instruction.