Children experience good and bad days in their performance. Although this phenomenon is well-known to teachers, parents, and students it has not been investigated empirically. We examined whether children’s working memory performance varies systematically from day to day and to which extent fluctuations at faster timescales (i.e., occasions, moments) contribute to daily WM fluctuations in the school context. In an ambulatory assessment study, Grade 3 and Grade 4 students (8 to 11 years old; N = 110) completed WM tasks on smartphones 3 times a day in school and at home for 4 weeks. Results showed substantial within-person fluctuations in children’s daily WM performance. Across task conditions, day-to-day, occasion-to-occasion, and moment-to-moment variability accounted for roughly the same extent of observed day-to-day variability with large individual differences in the amount of reliable fluctuations at the different timescales. Grade 3 students were more variable than were Grade 4 students at the faster timescales, more variable WM performance at all timescales was related to lower school achievement, and more day-to-day variability was associated with lower fluid intelligence. These findings build the foundation for research on the antecedents and consequences of children’s fluctuating cognitive resources. Theories about cognitive development and learning should consider performance fluctuations across and within days to understand the processes underlying long-term changes. Educational practice may be informed by the substantial WM fluctuations at all timescales and adopt interventions that increase children’s attentional focus and self-regulation.