We examine the impact of offering full-day as a replacement for half-day kindergarten on mothers' labour supply using the rollout of full-day kindergarten in Ontario, Canada. We find no impact on the extensive margin but do find one on the intensive margin. In particular, we find that access to full-day kindergarten increases weekly hours worked and decreases absenteeism for mothers of four-year-olds. We find that this effect is driven by specific sub-groups, namely non-immigrant mothers of only one child with low education levels who live in urban areas.