Monday
Monday, as a named concept, emerged approximately 2,000 years ago with the Roman adoption of the seven-day week. Prior civilisations organised time differently; the concept of a "first day of the work week" would have been incomprehensible to many ancient societies that operated on different calendrical systems.
Furthermore, Monday's continued existence depends entirely upon human civilisation maintaining its current temporal organisation. Should humanity adopt a different system - as has been proposed periodically throughout history - Monday would simply cease to exist as a meaningful concept.