This question already has an answer here:
In history the Romans didn't count their days of a month not by adding each day extra. But instead of that they used three moments in a month, Kalendae, nonae and Idus. And from those days they counted back. Now in the 4th century Constantine introduced with his new christian religion also a new kind of dating. The 8 days week was changed in a 7 days week. And also the names of the days changed from A t/m H to the planetairy names of the day, like the roman languages has at the moment.
But was that also the day that they start counting like 1,2,3...28,29,30 of the month?