Recent evidence suggests that the cerebellar hemispheres are in a unique position to contribute to motor planning via cortico-cerebellar loops. However, it is unknown how the dynamics of preparatory activity relate to the functional parcellation of the cerebellar cortex. We used mesoscale widefield calcium imaging to study molecular layer interneuron and Purkinje cell dendritic Ca2+ activity in Simplex, Crus I and Crus II during a targeted lick-interception task. We identified independent spatial sources of Ca2+ activity in interneurons and Purkinje cell dendrites using non-negative matrix factorization and spatial independent component analysis, respectively. Interneurons and Purkinje cell dendrites showed opposite ramping activity preceding the initiation of targeted licks by several seconds. Moreover, we uncovered motor preparatory signals in patches of interneurons in Crus I that cannot be explained by ongoing motor behavior and are absent during spontaneous behavior. As mice learned to initiate targeted licks with high temporal precision, the predictability of interneuron preparatory activity increased. Chemogenetic inhibition of interneurons in Crus I resulted in disruption of Purkinje cell dendritic Ca2+ activity and decreased temporal precision of licks. These findings suggest that coordinated preparatory activity across patches of interneurons in the cerebellar hemispheres are key to achieving optimal motor timing.