Flowers blooming, fungi fruiting, insects biting, fish spawning, geese migrating, deer calving; our consciousness is steeped in a seasonal calendar of nature's events. Phenology is the study of these recurring, seasonal life-history events, though nowadays this term is widely applied to the events themselves. From Shakespeare's sonnet 98, "From you I have been absent in the spring", to the appearance of seasonal events and migratory species in the oral traditions of Native Americans, interest in phenology is long-standing and transcends cultures. In this primer we introduce the study of phenology, trace the development of the field, and examine the prominent role phenology has played in evidencing the widespread impacts of anthropogenic climate change on life on Earth. We will consider the potential implications of climatic change for the ability of populations to persist and the stability of species interactions.