eli5 What is the concept of months? Why Feb has only 28 while some have 31? When did the concept of 365 days’ year come in people?
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A year’s duration comes from the seasons, which can be tracked by looking at the stars. Seasons are super important for agriculture–you wouldn’t want to plant a crop as winter is about to start–which is what spurred astronomers to develop a method for tracking the seasons accurately. Earth orbits once per 365.24… days, hence a 365 day year that adds an extra day roughly every 4 years.
Months are prompted by a desire to have a smaller unit of time that’s bigger than a day. The moon’s cycle offers a convenient and easily tracked period of about 29.5 days, so it influenced that period. Unfortunately the moon’s cycle is neither a whole number of days nor an even fraction of a year.
To understand the modern calendar it’s best to turn to Rome. The Roman calendar had, at one point, a mere 10 months. Roman culture preferred odd numbers to even ones, so these months had either 29 days (“hollow” months) or 31 (“full” months) (aside: I’m glossing over a *lot* of revisions of calendars here; some had 29 and 30 day months, some keyed directly off the moon, etc). These months were the equivalent to modern day March – December, which explains the names of Sept-December (sept, oct, nov, and dec are latin prefixes for 7, 8, 9, and 10, and these were the 7th through 10th months. Note that initially July and August were named similarly, as quintilis and sextilis, but they were later renamed after Juilus and Augustus. A popular myth is that July and August were inserted, but they were merely renamed).
That calendar’s start date made good sense from a seasonal perspective: it was keyed off of the spring equinox, towards the end of March. Note that the Romans counted dates in terms of the days until certain key dates of a month, as opposed to the modern system of counting up from the first. Even in modern times the spring equinox is in late March.
However, it doesn’t take much math to recognize that a year of 10 months, none with more than 31 days, is never going to get up to the 365.24 days needed for a full year. If you just had those ~300 days then the seasons would slip by a *lot* each year, to the point of making it useless for agriculture. To avoid this problem there was an additional unnamed “intercalary” month (literally “between years”), which was about 60 days long. It was up to certain government officials to determine when this month was over and it was time to declare the new year. Unfortunately, many political positions turned over with a new year, so that led to the duration of the intercalary month being unreasonably short when opponents were in power, or unreasonably long when allies were in power.
That system was tossed in the trash and two new months were added. Not July and August, as many may claim, but January and February. In some calendars these were tacked on as the 11th and 12th months of the year, while in others they were the 1st and 2nd. Having February as the last month of the year explains why it gets “whatever is left over to hit 365” days, which turns out to be 28-29. However, it ultimately settled in as the 2nd month of the year over the course of many calendar reforms.
Those calendar reforms ultimately moved away from Romans’ 29 and 31 day months in favor of 30 and 31 day months, still with February as the month that gets what’s left over. And that’s how we wound up with a calendar that starts on an unremarkable day in the middle of winter, with months that schoolchildren have to learn a mnemonic to remember the duration of, and with the second month of the year being the oddball that has to pick up the slack to keep the year from drifting.
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