We examine the effects of offering full-day kindergarten as a replacement for half-day kindergarten on mothers' labor supply using the rollout of full-day kindergarten in Ontario, Canada. We find no effect on the extensive margin but found an effect on the intensive margin. In particular, we find that access to full-day kindergarten increases weekly hours worked and decreases absenteeism among mothers with kindergarten-aged children. This effect is driven by specific subgroups, namely, nonimmigrant mothers with low education levels who live in urban areas and have only one child.