Four-day school weeks are used in over 1,600 schools across twenty-four states but little is known about adoption and implementation of these types of school calendars. Through examinations of school calendars and correspondence with school districts, we have compiled the most complete four-day school week dataset to date. We use this unique database to conduct a comprehensive analysis of four-day school week policy adoption and implementation. We find adoption of four-day school weeks is often financially motivated and has generally remained a small, rural district phenomenon. These schedules feature a day off once a week—often Friday—with increased time in school on each of the remaining four school days that, on average, is nearly an hour longer than the national average among five-day schools. Four-day school week schedules average only 148 school days per year, resulting in less time in school than the national average for five-day schools (180 days per year) despite the longer school days. Substantial heterogeneity exists in the structure of these schedules across states, which may help explain differential four-day school week effects on student outcomes across institutional settings in the previous literature.