What was the Julian Calendar aligned to?

Question

I understand that the Julian Calendar was introduced to align the length of the calendar to the tropical year, i.e. make the average year 365.25 days long. However, what I'm still trying to figure out is that were they trying to align the start of the calendar with? Or, put it another way, why 1 January is not aligned to a solstice, equinox or some other astronomical event?

Wikipedia currently states (emphasis mine):

The first step of the [Julian] reform was to realign the start of the calendar year (1 January) to the tropical year by making 46 BC (708 AUC) 445 days long, compensating for the intercalations which had been missed during Caesar's pontificate. This year had already been extended from 355 to 378 days by the insertion of a regular intercalary month in February. When Caesar decreed the reform, probably shortly after his return from the African campaign in late Quintilis (July), he added 67 more days by inserting two extraordinary intercalary months between November and December.

Realign the start of the year to what of the tropical year? A footnote further explains:

It is not known why he decided that 67 was the correct number of days to add. Ideler suggested (Handbuch der mathematischen und technischen Chronologie II 123-125) that he intended to align the winter solstice to a traditional date of 25 December. The number may compensate for three omitted intercalary months (67 = 22+23+22). It also made the distance from 1 March 46 BC, the original New Years Day in the Roman calendar, to 1 January 45 BC 365 days.

But why was the date 25 December special or traditional at that point in time?

Answer 1

Possibly they wanted to match it to Brumalia. The Roman winter solstice festival. wikipedia: "The Brumalia was also celebrated during the space of thirty days, commencing on 24 November and ending with the "Waxing of the Light", December 25" citation

Much the same can be said about Saturnalia, they're very similar. The "Birthday of the Unconquered Sun" is another Decemeber 25th shortest day thing, but it's usage seems to be late roman empire.

The tropical year is the solar year. It's a measurement of the position of the sun. It was probably based on Hipparchus's work on equinoxes.

Also the choice of this date is to do with Pliny: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D18%3Achapter%3D59

Answer 2

Sol Invictus was said to have died and gone to hell on the Noctis, and three days later rose from the grave on that day, celebrated in the Roman festival of Dies Natalis Solis Invicti, or "Birthday of the Unconquered Sun". It's my own understanding from research and reading the material that this is why Christmas is celebrated on that day, although, as you can read in the link below this position isn't held by everyone.

Wikipedia