The authors examined whether variations in day-to-day estimates of personality characteristics, which are often treated as error, are instead predictable and meaningful. Using event-sampling and spectral analysis, they found that variations in interpersonal behavior over weekly periods were cyclic and normative. Dominant, submissive, agreeable, and quarrelsome behaviors rose during the week and fell on the weekend. The more general dimensions of agency and communion exhibited opposite patterns of cyclicity, with agency rising and communion falling during the week. Interpersonal traits were not useful in predicting behavior cyclicity. Extraverts exhibited a daily cycle, partially mediated by more varied partners and social behaviors during evenings. Findings are discussed with reference to conceptions of personality expression as dynamic yet stable processes.