When was February 29th instituted in our modern calendar and how did they figure out that we needed one extra day every four years? How did people account for this before hand?
The idea of adding an extra day to February dates back to Julius Caesar's reform of the Roman calendar in 46BC.
Before Julius Caesar, the Roman year was 355 days long, based on 12 months of 29 and 31 days (seven 29-day months and five 31-day months). Every second year, they added an intercalary month of 22 days during the month of Februarius to make up the difference.
Except... the responsibility for adding the Mensis Intercalaris fell on the College of Pontifices - the College of Priests of Rome. And, they were all sorts of unreliable, ranging from distracted and forgetful to political and malicious. Some years they simply forgot to add the Mensis Intercalaris, some years they were distracted by things like wars, and other years they deliberately didn't add the leap-month because they didn't like the consuls of the current year, so didn't want to give them an extra 22 days in office. After a few centuries of this, the Roman calendar was quite out of line with the seasons (when Caesar fixed things up, he had to add 90 days to one year to fix things up).
Caesar had spent some time in Egypt the year before his calendar reforms in Rome. (He couldn't have been there for much longer than a month, but Cleopatra gave birth to a son about nine months later who she called Caesarion, and who she claimed was Caesar's son. Appian wrote two hundred years later that "[Caesar] made a voyage on the Nile to look at the country with a flotilla of 400 ships in the company of Cleopatra, and enjoyed himself with her in other ways as well." Caesar never officially acknowledged the boy as his son, but never denied Cleopatra's story, either. Ahem.) The Egyptians had been using a strict 365-day year for thousands of years before this time. It fell one full day short of the seasons every 4 years, and cycled through the seasons every 1,460 years. (Over that millennium-and-a-half period, the New Year would gradually get earlier and earlier compared to the seasons until it eventually ended up back where it started!) A couple of hundred years before Caesar's time, the Ptolemaic rulers of Egypt had decreed that every fourth year should have an extra day added to it to keep the calendar aligned with the seasons, but the Egyptians weren't having a bar of it: the Egyptians knew when the seasons where, regardless of what the calendar might say, and didn't need some Greek kings who had only been in Egypt for a few decades telling them any different. Anyway, Caesar probably met some officials and priests on his tour of Egypt who told him about leap days.
When he returned to Rome, he instituted a similar system there: he changed the number of days in various months to increase the total to 365 days, he removed the Mensis Intercalaris, and he added a leap day to Februarius every four years.
Interestingly, this leap day was not added to the end of the month, but after the twenty-third day of Februarius: the Terminalia. This was probably because of how the Romans referred to days. They didn't number them like we do, they referred to them as being certain number of days before or after given key dates in each month: the Kalends, the Nones, and the Ides. The Kalends was the first day of the month. And, the last few days of the prior month were referred to by counting down to the next Kalends. For example, the twenty-fourth day of Februarius was referred to as "the sixth day before the Kalends of Martius". So, the last date with a "Februarius" name was the Terminalia: the tenth day after the Ides of Februarius. The leap day was inserted after Terminalia.
Anyway, that's when a leap day was added to February: 46BC, by Julius Caesar.
As for when that leap day was officially made the 29th of February, that occurred with the Gregorian calendar reform about 1,600 years later. While Pope Gregory was adjusting the calendar in 1582 AD to remove three leap days every four centuries, he confirmed that the extra day being added every fourth year was the 29th day of February.
hi! there are a few threads discussing the evolution of calendars in the FAQ*
*see the link on the sidebar or the wiki tab
I believe Gaius Julius Caesar of the patrician gens Julii instiuted it along with moving October from the eight month (octo is latin for eight) to the tenth month to make way for July and August. This was in the Julian calender.