Interest in year-round schooling is motivated by international comparisons of time spent in-school and efforts by policymakers to identify viable policy avenues for improving achievement and reducing costs. Prior research on effectiveness of modified year-round schools finds modest support for a modified year-round calendar, but much of the research is weak. Both memory and time-on-task literatures provide a framework for understanding how patterns of schooling and non-schooling intervals could impact student learning. While there is some evidence that the learning losses from summer breaks are greatest for students of low socio-economic status, there are few inquiries into the effects of year round schooling on these students, or other important student subgroups such as English language learners and students in special education. Using an extensive micro-level longitudinal database I compare the achievement of students under a traditional or modified year-round calendars. Capitalizing on a natural experiment in Wake County, NC wherein schools were switched from a traditional to a year-round calendar, I apply a student fixed effects method to isolate the effect of calendar arrangement on student achievement and student absenteeism. To complement the student fixed effects analysis and to increase the study's external validity, I use a growth curve analysis to compare outcomes for students attending a modified year-round calendar to students attending similar schools operating under a traditional calendar. In addition, I examine whether the modified year-round calendar is advantageous for increasing retention and reducing costs. There were five major findings in this dissertation. First, the modified year-round calendar leads to improved student achievement for students of low socio-economic status and second, the modified year-round calendar is also beneficial for students with special needs. Third, the modified year-round calendar is detrimental to student performance for students who are English language learners. Fourth, the link between the modified year-round calendar and lower rates of student absenteeism is supported in the student fixed effects methodology. Fifth, higher teacher retention rates are correlated with a change from a traditional calendar to a modified year-round calendar. Future research efforts are suggested including an investigation of potential mediators for the modified year-round calendar effect.