Monday is sometimes held to be the first day of the week (especially in modern Europe, see ISO 8601), and sometimes the second day (a traditional view derived from ancient Jews and still standard in the United States).
Modern culture usually looks at Monday as the beginning of the work week, as it is the day children typically go back to school after the weekend, and the day adults typically go back to work after the weekend. It is speculated, amongst some other reasons, that the colloquial "illness" Mondayitis is so named. Another reason for Mondayitis is speculated to be that human circadian rhythms are incompatible with the normal 24-hour working week.
In the popular rhyme, "Monday's Child is fair of face".