Why do January, March, May, July, August, October, and December have 31 days {calendar, western}. Why do April, June, September, and November have 30 days? Why does February have 28 or 29 days? The current calendar keeps solar year and months synchronized and puts leap-year day in winter.
orbit
In Northern Hemisphere, Earth orbit makes summer longer than winter, so days of months make summer longer and keep equinoxes at approximately same date.
From December 21 to March 21 are 90 or 91 days. From March 21 to June 21 are 92 days. From June 21 to September 21 are 92 days. From September 21 to December 21 are 91 days. Therefore, in Northern Hemisphere, fall and winter have 181 or 182 days, and spring and summer have 184 days.
Leap-year day must be in winter because summer is less than exactly three days longer than winter.
different calendar
January, March, May, July, September, November, and December can have 30 days, and February, April, June, August, and October can have 31 days, to make 365-day year. To make 366-day year, December has 31 days.
However, then fall and winter have 91 + 91 = 182 days, which is too much. January, February, March, September, October, November, and December can have 30 days, and April, May, June, July, and August can have 31 days, to make 365-day year. To make 366-day year, December has 31 days. Then, fall and winter have 90 + 90 = 180 days, which is too few.
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Date Modified: 2022.0224