This study addressed two questions: Do daily fluctuations in mood exhibit a 7-day (circaseptum) cycle, and are there reliable individual differences in how entrained people's moods are to such a weekly cycle? Spectral analysis of daily mood over 84 occasions revealed a strong weekly rhythm in the temporal organization of mood in a sample of 74 undergraduates. A sine wave with a period of 7 days accounted for 40% of the variance in the daily mood data. Individual differences were also found in how entrained subjects' moods were to this weekly rhythm. We predicted that extraverts (compared with introverts) should be less entrained to a weekly cycle. Results suggest that the novelty- and sensation-seeking behavior of extraverts most likely serves to lessen the cyclical predictability of their day-to-day moods. The origin and psychological meaning of the 7-day week are discussed.