Motivation is known to fluctuate, often declining, over time. Research on how motivation changes across school years is ubiquitous, yet little research examines changes in motivation within one school year. In this study, we model how third- through fifth-grade students' mathematics motivation (i.e., expectancies and values) changes over two years using six time points (beginning, middle, and end of school years). We compared a model forcing a linear trend to an unstructured model allowing for trends in motivation to take any shape. On average, mathematics expectancy and value both decreased linearly over the two years; however, the unstructured model for expectancy displayed better fit and showed more variation, with motivation often peaking at the beginning of the school year and dipping lowest in the middle. This mid-year dip offers insights into contextual forces that may influence student motivation and provides information that can be used in structuring motivation-supporting interventions.