Seasonality is presented as a multi-disciplinary phenomenon that transcends mere statistical fluctuations. It is observed in nature through the survival journeys of wood frogs, the transmission of malaria, and the great migrations of geese and wildebeests. Indigenous societies, such as the Inuit and Sámi, demonstrate a lived attunement to these periodic rhythms, organizing their social and economic lives around environmental cues rather than formal calendars. Historical analysis reveals that ancient civilizations like the Egyptians and Romans struggled with calendar drift and seasonal constraints in food logistics and public health. The linguistic evolution of “season” from its agricultural roots in Latin underscores its long association with functional appropriateness and natural cycles. Early economic thinkers like Adam Smith and William Stanley Jevons recognized the significance of seasonal variation in prices and agricultural output. Within modern tourism, seasonality is revealed as a complex, multi-layered force driven by climate, institutional calendars, and behavioral factors. Understanding these dynamics requires a comprehensive economic framework to address market inefficiencies, temporal shifts in demand, and the strategic adaptation of destinations. By integrating biological, historical, and linguistic perspectives, this chapter establishes seasonality as a foundational concept for understanding the structured yet dynamic flow of global tourism systems.