Every other year, however, the pontiffs intercalated another month in February. It was somewhat complex because Intercalaris could have 22 or 23 days and February itself would be shortened to 26 days. What this meant was that the sum of the two would be 48 or 49 days, and the year would be 375 or 376 days long.

In short, two consecutive years in the pre-julian calendar would have a total of 730 or 731 days. Divide that by 2 and you have a normal 365 or 366 days year.

So? Where's the problem? Nowhere. This calendar served the Romans very well for hundreds of years. Except that in times of political or military trouble the pontiffs might be unable to make the necessary intercalations and the year would then quickly become out of sync with the seasons. Getting it back in order would then be very troublesome.

This happened when issues became serious between the Senate and Pompey on one side, and Caesar himself on the other, and through the ensuing civil war. Intercalaris was not intercalated when it should have been in 50 BCE or on the following years. This led to spring beginning later by ten days every year. In 50 BCE the equinox was in early April, in 49 BCE in mid May, and by 46 BCE winter started after mid February.

This is when Caesar intervened as high pontiff. 46 was "the last year of confusion". His reform was to give aditional days to the months so as to have every year last 365 days, with an additional day in February every four years. But to bring the calendar again into sync with the seasons he had to intercalate three months before December of 46 BCE (I believe all of them between November and December, although some think one was at the proper time in February), totalling 67 days, and making this year 445 days long.
The winter solstice of 46 BCE fell on the 21st of December. The Kalends (1st day) of January fell on the first new moon after the solstice. Everything was put right again.

And everything would work fine without pontiffical intervention, so that everyone would know what date it was and could plan sowing and harvesting and war and all the business that depended on the seasons. It would be fine forever. Or so Caesar thought.

His contemporaries, when they were not mumbling and ranting against his excessive power ("The constellation Lyra will be up tomorrow? Certainly, it has been ordered so", was Cicero's reaction), understood how this reform simplified life, and the Senate decreed that the month of Caesar's birthday, Quintilis, would thereafter be named Julius after him. I believe this was either decreed after Caesar's death or took effect only after his death, because in 44 BCE Brutus was furious when he realised the Apollo games he was about to stage were advertised for July and not Quintilis!

Later the julian calendar had to be adjusted. The first adjustment was made by Augustus (who was high pontiff) in the year 8 BCE, because of the fact that Romans counted inclusively, i. e., the first and last year of a period belong to that period, and the last year is also the first of the next period and gets counted again. So the bissextile year which should be every fourth year was really the third - and the calendar was moving too slowly!
The Senate decreed that the month of Sextilis, a lucky month for Augustus, would be named after him.

After that no more adjustments until Pope (high pontiff) Gregory XIII's in 1582 because by then there were 10 days too many and the equinoxes were again out of sync. This was seventeen centuries after Caesar. No more adjustments are expected for the next three thousand years.

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