The quantitative, causal-comparative study posed two research questions: (1) What is the relationship between the type of academic calendar and achievement in reading and mathematics for students from low socioeconomic backgrounds? and (2) Does the type of academic calendar affect the amount of summer learning loss experienced by students from low socioeconomic backgrounds. The sample was comprised of 33 year-round schools and 33 traditional calendar schools in a Southeastern state where at least 40% of the student populations were eligible for the federally supported free or reduced-cost lunch program. Archival achievement data were analyzed using an independent samples t test to determine the statistical significance of the relationship between the academic achievement of students from year-round and traditional calendar schools. The summer learning loss experienced in year-round and traditional calendar schools was measured by calculating improvement scores and compared using an independent samples t test. Study results indicated that school calendars did not significantly influence academic achievement or summer learning loss among fourth grade classes from economically disadvantaged schools in a Southeastern state. These results implied that both year-round and traditional school calendars are equally valuable in terms of academic outcomes. This work contributes to the body of knowledge on school calendar modification and the socioeconomic achievement gap. The theoretical foundation for this study was provided by the spaced learning effect because it addressed the potential of spaced learning intervals to increase the rate of students' information retention.