Background: School district implementation of a four-day school week (4DSW) calendar has skyrocketed in recent years, despite little evidence regarding the impact on student and teacher outcomes. Between 1999 and 2019, the number of 4DSW districts increased by over 500% (Thompson et al., 2021). Since 2019, the rate of 4DSW adoption has increased even more. In Missouri, the setting for the study herein, the number of 4DSW districts -- mostly small and rural -- increased by over 300%, with over 30% of the state's 500+ districts operating a 4DSW schedule. There is still much to learn about the impacts of a 4DSW on students, teachers, and communities, however a growing body of literature finds mixed, yet mostly negative, effects on student achievement (Thompson & Ward, 2020; Morton et al., 2022; Thompson et al., 2022; Anderson & Walker, 2015; Morton, 2023; Morton, 2021). Furthermore, districts that adopt the 4DSW 1) serve significantly higher proportions of FRPL-eligible students than 5DSW districts (Thompson et al., 2021) and 2) are primarily small, rural districts that face unique staffing challenges (Thompson et al., 2021, Nguyen, 2020). It is therefore critical to advance our knowledge of the effects of a 4DSW to better inform policymakers, practitioners, families, and scholars. Prior to the COVID-19 pandemic, the most cited reason for adopting a 4DSW was financial pressure with teacher retention and other challenges particularly salient to rural districts as the second most cited reason (Thompson et al., 2021). Indeed, rural districts struggle to match salary offerings and lack other non-pecuniary benefits associated with non-rural areas (e.g., access to healthcare, amenities, etc) (Nguyen, 2020). Therefore, the 4DSW may be appealing to teachers and aid rural districts in keeping high-performing teachers, especially early-career teachers. More recent, post-pandemic, data suggests that teacher recruitment and retention has become the leading rationale district leaders cite for implementing a 4DSW, particularly in small, rural districts (Office of Education Policy, 2023). Despite this rationale I am aware of no study that examines the causal effects of the 4DSW on teacher turnover or retention. This study seeks to fill this crucial gap. Purpose: As mentioned above, rural districts may struggle to match salary offerings, lack other non-pecuniary benefits associated with non-rural areas, and face unique staffing challenges in comparison to their urban counterparts (Nguyen, 2020; Goldhaber et al., 2020); therefore, rural districts have been implementing 4DSWs as a teacher retention tactic. Empirical evidence finds that teachers are less likely to turnover in schools with better working conditions (Nguyen et al., 2020); to the extent that teachers perceive a 4DSW as an improved working condition, it would be expected that turnover decreases because of policy adoption. However, if a 4DSW is not enough to retain or attract teachers, then turnover would not be affected. This study answers the following research questions: (1) What is the effect of the 4-day school week on teacher retention? (a) To what extent does this effect vary across teacher demographics, including teacher race, gender, and years of experience? Setting and Population: This study examines 4DSW adoption in Missouri. As of the 2022-23 academic year, 144 school districts in Missouri -- nearly 30% of the state's districts - are using the 4DSW calendar, with even more slated to adopt the policy in the 2023-24 academic year. These districts are predominantly rural, though at least one urban district in the state has adopted the 4DSW. Panel data from 2005-06 through 2021-22 will come from the Missouri Department of Secondary and Elementary Education (DESE) and include 1) teacher-level administrative data and 2) student-level achievement and demographic data. Methods and Data: I will use difference-in-difference (DID) and time series models to answer the research questions. The primary DID model is as follows: y_isdt=??FDSW?_dt+A_isdt ?+B_st ?+C_dt ?+?_d+?_t+?_ist where y_isdt is an indicator of teacher turnover equal to 1 if teacher i in school s in district d in year t is not in the same district in year t+1. ? is the coefficient of interest and ?FDSW?_dt is an indicator variable equal to 1 in the year district d implements2 the FDSW. I will control for: teacher-level characteristics (A_isdt) including race, gender, education level, subject, and years of experience; school-level characteristics (B_st) including prior year teacher turnover, proportions of student race, gender, FRPL, ELL, gifted, and special education status; and comparable district-level characteristics (C_dt). ?_d are district fixed effects and ?_t are year fixed effects; ?_ist is the error term. An event study analysis will also be conducted, including the same terms but will estimate yearrelative-to-treatment point estimates rather than pooling post-treatment effects. Finally, due to the staggered timing of treatment, I will also test robustness of results to alternative DID estimators per de Chaisemartin & D'Haultfeuille (2020), Callaway & Sant'Anna (2021) and others.