Ever since the 1983 landmark study, "A Nation at Risk", was released, educators have been examining the effectiveness of instructional time in American schools (Pedersen, 2012). However, school calendars have remained stagnant and seemingly a product of society and economy, tied to agriculture, for over 100 years (Turner & Finch, 2018). In Missouri, a growing number of schools are reevaluating the five-day school week and implementing a four-day school week in response (Turner & Finch, 2018). As stated by Thompson (2020), the impact of the four-day school week on student achievement requires more study. This study involved investigating the impact of the four-day school week on student academic achievement by examining 7th-grade and 8th-grade MAP data provided by the Missouri Department of Elementary and Secondary Education in the areas of math and ELA prior to and following the implementation of the four-day school week. Additionally, the perceptions of principals and teachers who work within the four-day school were collected to examine teacher morale, school finance, student discipline, and overall attendance (teachers and students). Quantitative data were analyzed to investigate pre- and post-implementation patterns. Furthermore, qualitative data in the form of interview responses were analyzed using coding methods to identify common phrases, keywords, and themes. The study findings revealed the four-day school week produced a negative impact on 7th-grade ELA, according to MAP data analysis, whereas 7th-grade math, 8th-grade ELA, and 8th-grade math indicated an insignificant impact. Additionally, the perceptions of principals and teachers indicated the four-day school week was an overall benefit to the school climate. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]