Monday is the day of the week between Sunday and Tuesday. It gets its name from the Mona, the Saxon Moon god.
Monday is sometimes held to be the first day of the week. This is the case in most of Europe and South America. In Asia, many languages refer to Monday as "day one". For example, Monday is "Xingqi Yi" in Chinese. Xingqi means day. Yi means one. The international standard ISO 8601 also defines Monday as the first day of the week.
In other areas of the world, Monday is the second day. This is the traditional view in the United States of America. The name for the day in Arabic, Greek, Hebrew and Portuguese is "second day". Quakers also traditionally refer to Monday as "Second Day" eschewing the "pagan" origin of the English name "Monday".
Modern culture usually looks at Monday as the beginning of the workweek, as it is typically Monday when adults go back to work and children back to school after the weekend, hence the colloquial "illness" Mondayitis. Also known as having a case of "The Mondays." A possible reason for Mondayitis is that human circadian rhythms are incompatible with the normal 35 to 40-hour working week.
In the popular rhyme, "Monday's Child is fair of face".