Although the four-day school week originated in 1936, it was not widely implemented until 1973 when there was a need to conserve energy and reduce operating costs. This study investigated how achievement tests scores of schools with a four-day school week compared with schools with a traditional five-day school week. The study focused on student performance in Colorado where 62 school districts operated a four-day school week. The results of the Colorado Student Assessment Program (CSAP) were utilized to examine student performance in reading, writing, and mathematics in grades 3 through 10. While the mean test scores for five-day week schools exceeded those of four-day week schools in 11 of the 12 test comparisons, the differences were slight, with only one area revealing a statistically significant difference. This study concludes that decisions to change to the four-day week should be for reasons other than student academic performance. (Contains 1 table.)