This study examined possible differences between a traditional and a year-round academic calendar with regard to student achievement in two suburban high schools in northern Virginia. This study’s importance derives from the fact that additional exploration is needed in order to more fully explore the potential loss of learned knowledge and diminished retention and recall of instructional material in students who experience an extended summer vacation period as part of a traditional school calendar. The purpose of this non-experimental, ex post facto, causal-comparative study was to test the theory of German psychologist, Hermann Ebbinghaus, called the spacing effect as it related to a comparison of the traditional school calendar and a year-round school calendar. While existing research has suggested that a traditional calendar results in summer learning loss, the current study did provide some support to the assumption that a summer learning loss had a long term impact on the standardized test performance of students. In order to further examine this finding, a group of test scores from 100 seniors from each high school (for a total of 200), who shared similar demographic characteristics, were used to create the data set for the study. All the seniors from these two separate but similar high schools (located less than four miles apart) were required to take the end of year (EOY) Virginia standards of learning (SOL) test near the end of their senior year of high school. The SOL must be taken and passed in the spring of the senior year in order for a senior to graduate from high school. These schools employed different school calendars. One adhered to a traditional calendar, the other a year-round calendar. The Virginia Standards of Learning (SOL) assessment is a timed test comprised of four comprehensive sections (math, science, reading, and social studies). This study used a non-experimental, causal-comparative (ex-post facto) research design with the independent variable being split into two non-manipulated groups (one traditional calendar and one year-round calendar), and the dependent variable being the pre-existing SOL test scores of the participants. Data were evaluated using independent-samples t-tests. Suggestions for further research which would enhance the validity of this area of research are discussed.