We present a new statistic called the “Mean Symmetrized Residual” (MSR) for detection and quantification of a weekly cycle in measured daily atmospheric carbon dioxide (CO₂). At the Mauna Loa Observatory in Hawaii, we conclude that CO₂ concentrations, on average, are significantly lower (0.022 parts per million by volume, ppmv) on weekends (Saturday–Sunday) than during the rest of the week. Over the past twenty-five years, the variation of the mean values of MSR (as a function of day of the week) has been relatively stable. We speculate that the observed weekday/weekend variation in CO₂ at Mauna Loa is the result of anthropogenic emissions on Hawaii and nearby sources. We do not detect a weekly cycle in daily CO₂ concentration measured at South Pole, Antarctica. This methodology has applicability to a variety of datasets.