For example, many may call today "Monday". How was it decided that a certain day would be Monday? When was this system put into effect, and who was the person/group who put this system into effect?
The modern days of the week are from the Roman calendar, just as are the months. The days of the week generally correspond to gods/planets but the words we use in English are the German equivalent god's for the Roman god's originally used. For example Thursday is Thor's day but in latin it's named after Jupiter as Jovis.