Why is January 1st the beginning of a year?

Why on earth is January 1st the first day of a year? A year in the solar calendar is defined as (roughly speaking) the time it takes for the earth to go round the sun. If so, why doesn't a year start on, say, the spring equinox or the winter solstice?

A bit of Google Search reveals the following facts.

Around 753 BC, Romulus, the founder of Rome, created a solar calendar that indeed began on the spring equinox. He created 10 months of 30 or 31 days, starting from March to December. (This is why September originally means the 7th month in Latin, October 8th, and so forth.) Interestingly, he left the remaining winter days after the end of December unassigned to any month. In those days, winter days were probably of irrelevance as nothing could grow. What turns out to be important is that the names of the first four months (March to June) are derived from Roman gods.

Around 713 BC, Numa Pompilius, a king of Rome, reformed the calendar, adding January and February after December. These two names also originate from Roman gods. Especially, January refers to Janus, the god of doors.

Then came Julius Caesar. In 46 BC, he reformed the calendar to create what is now known as the Julian calendar. Famously, he gave his own name to the month after June, that is, July. (And then the succeeding Roman Emperor Augustus, jealous of Caesar, gave his own name to the month after July, that is August.)

Up to here, the source of information is the Wikipedia article on Roman calendar (as of 31 December, 2012).

Now, lesser known is the fact that Caesar declared that January was the first month of the year because it was the month dedicated to Janus. As mentioned above, Janus is the god of doors, which symbolize the beginning and the transition. He has two heads, one looking to the past and the other to the future. Caesar thought Janus should be the god for the first month of a year.

Janus on a Roman coin from the 2nd century BC (borrowed from here).

After the demise of the Roman Empire, each region of Europe adopted different dates of the New Year's Day (e.g. Christmas Day, Easter Sunday). Overtime, however, they converged to January 1st, mostly when they adopted the Gregorian calendar, the calendar that Pope Gregory XIII devised by reforming the Julian calendar and that most of us in this world use today.

The last two paragraphs are based on Matt Soniak "Why Does the New Year Start on January 1?" Mental Floss, December 31, 2011.

So today we (excluding those in Saudi Arabia, Ethiopia, Iran, and Afghanistan according to Wikipedia on civil calendar) celebrate the New Year's Day on a day when the Earth positions itself in a particular, rather random, location on its orbit around the Sun, for the reason based on Roman mythology and an idea that probably just popped into Caesar's head.