The aim of this research paper is to present a comprehensive dictionary-based linguistic analysis of the English word day, focusing on its etymology, historical development, semantic evolution, and usage across different linguistic and cultural contexts. Although day is among the most common and seemingly simple words in English, its long historical continuity and semantic flexibility make it a valuable subject for lexicographic and linguistic investigation. Drawing on authoritative sources such as the Oxford English Dictionary, the Online Etymology Dictionary, and comparative Indo-European studies, the paper traces the word’s development from its Proto-Indo-European origins through Proto-Germanic and Old English forms to its present day meanings in Modern English. The study examines how the Old English term dæġ, derived from the Proto-Germanic root dagaz, initially referred primarily to daylight hours and later expanded to include a full 24-hour period. This semantic expansion is shown to have occured alongside social, cultural, and religious changes in time measurement, particularly during the Anglo-Saxon period. Despite these changes, the core meaning of the word day, as a unit or division of time has remained stable across centuries. This paper explores the word’s polysemy by analyzing its idiomatic and figurative uses, including expressions such as day off, all in a day’s work, or one of these/those days, which demonstrate how temporal meaning extends into social and emotional domains. In addition, contextual interpretation is discussed through religious texts, numeral modifiers, and modern informal usage, illustrating how meaning is shaped by pragmatic and cultural factors. The findings demonstrate that the word day exemplifies both linguistic continuity and semantic adaptability. By examining a single, everyday lexical item in depth, this paper highlights the importance of dictionary research and historical linguistics in understanding how language evolves while preserving essential meanings across time and communicative contexts.