Objective: We decompose the change in weekend childcare time into two parts, structural changes (such as an increase in the number of college degree–holding parents), and nonstructural changes (changes in behavior), to examine why the educational stratification in weekend childcare has widened. Background: In Japan, as in other developed countries, parental time spent on childcare has increased. This is because parental childcare has come to be considered an investment that enhances children’s abilities and hence, their future income. Our descriptive analysis shows that there was a disparity in the growth rate of weekend time spent on childcare after examining time spent on childcare in terms of the parents' combined educational backgrounds in the last two decades. Method: Using the 1996 and 2016 instances of the Japanese time use study, we analyzed not only total parental childcare time but also its components (maternal and paternal solo childcare time and coparenting time) via estimated OLS regressions and twofold Blinder–Oaxaca decompositions. Results: The results showed that the increase in weekend childcare time over the past 20 years was mainly explained by the overall change in parental behavior. The detailed decomposition results revealed that the behavioral changes on Sunday varied by assortative mating, with highly educated homogamous and hypogamous couples giving more enthusiasm and time to raising their children in a significant departure from other couples (especially less educated homogamous couples). Additionally, highly educated homogamous couples were more egalitarian in their sharing of childcare, with fathers spending more time in solo childcare and coparenting, while this was not the case for other couples. Conclusion: Educational disparities among parents who spend weekend time on childcare may reproduce this inequality throughout a child’s development. This concerning tendency might have even more acute effects if paternal childcare and coparenting have a positive impact on child outcomes.