Significance To do the right thing at the right time, organisms need to glean cues from their environment. How they respond can then be described by reaction norms, i.e., by the relationship between the phenotype expressed (the phenology of an event) and the environment (the date when a given number of degree-days are achieved). We use information on 178 phenological events across the former Soviet Union. We found the timing of events to differ more between sites in spring and less in autumn. These patterns of local adaptation translate to a massive imprint on nature’s calendar: geographic variation in phenology is more pronounced in spring and less pronounced in autumn than if organisms were to respond equally everywhere.