Employment rates are relatively lower for mothers with young children in developing countries. This paper analyzes how preschool attendance affects maternal labor supply in Argentina. Using pooled household surveys, we show that four year-olds with birthdays on June 30 have sharply higher probabilities of preschool attendance than children born on July 1, given enrollment-age rules. Regression-discontinuity estimates using this variation suggest that preschool attendance of the youngest child in the household increases maternal labor supply in the intensive and extensive margins.