What is the calculation principle of calendar period?

I didn't consider this estimate at first, because there are seven days in a week. If the whole week is regarded as a year, does it mean that the days in this week that do not belong to this year are also counted as this year? Then in this case, there is no way to achieve the original 365 days a year and one more day in leap years.

At the same time, the week is mainly convenient for us to form and calculate the schedule, and has little effect on calendar timing.

Therefore, many festivals in the United States refer to a certain day of a week in a month, not a certain day of a week.