In Texas, the adoption of a four-day school week (4dsw) calendar from a five-day school week (5dsw) has not been widespread until recent years, but has grown significantly to 40 districts post-COVID through the 2022-23 school year. This report presents findings of an analysis of State of Texas Assessments of Academic Readiness (STAAR®) performance in reading language arts (RLA) and mathematics comparing students from Texas public school campuses with a 4dsw calendar to students from Texas public school campuses with a 5dsw in the 2022-23 school year. For the analysis in this report, district-reported calendar data was used to estimate the number of campuses operating a 4dsw calendar, given districts do not register their status as 4dsw or 5dsw with the Texas Education Agency (TEA). After using multiple methods to confirm 4dsw calendar status for inclusion in the analysis, the sample included 137 campuses representing 76 districts. It was found that in 2022-23, 4dsw students had lower performance than 5dsw students in Grades 6-8 STAAR-RLA and Grades 4-6 STAAR-Mathematics. These differences were significant in a logistic regression that controlled for prior-year achievement scores. The percentage of students at 5dsw campuses who met the STAAR Meets Grade Level standard in RLA was 6-8 percentage points higher on average than students attending 4dsw campuses, and the percentage of students at 5dsw campuses who met the STAAR Meets Grade Level standard in mathematics was 5-8 percentage points higher on average than students attending 4dsw campuses.