How the person who made the calender know what day it was

The calendar was not invented, it evolved.

Remember, the “month” is basically the period of the moon. That’s why months are about 30 days; the period from new moon to new moon is about 28 days.

The phases of the moon last about 7 days—there’s your week.

If I remember correctly, they found a bone in Czechoslovakia that was about 27,000 years old with what look like tally marks for a month.

So, as numbers evolved (and the most primitive number system is 1, 2, many), the weeks and months evolved. Months evolved as they reflected the seasons, which governed the agricultural year.

Finally, the Romans gave us our current names for the months. The year was reset by the Gregorian calendar in the 16th Century. It had to be reset as Easter was drifting away from what it should be. The Gregorian calendar took out 3 days every 400 years, and renumbered the days of 1 month to fix it. This was done by different nations at differernt times; for example, Russia did not adopt the Gregorian calendar until after the October Revolution, which is why it took place 7 November 1917!

By the way, everyone is so used to thinking the world is on one date. However, different calendars are used by the Moslem, Jewish, and Bahai faiths even today. Check out the Wikipedia article on the various calendars.