Why the year begins on January 1?
The Spanish are the guilty, yes
Even solstices and equinoxes mark adequate to indicate a moment when it begins the year...
The beginning of the year on January 1 is related to the Romans traditionally marked the beginning of the year according to the time when consuls took up his duties, which occurred in early spring, and the fierce resistance of Hispanics, who made the Roman legions conquered 200 years will bring the Iberian Peninsula. In Babylon also began the year with the spring.
At one time the Romans realized that they needed to appoint its consuls with time for these were in office in early spring, ready to launch military campaigns with good weather, which was to bring forward its nomination more two months, bringing the beginning of the year was set in January.
However, the Roman calendar in the age of Julius Caesar was still a mess, it was ten months totaling 355 days, which occasionally intercalary months had to enter to set the date of the calendar with the actual date, so he decided to establish the Julian calendar did not need these intercalary months.
Isn´t known exactly why it did not align the January 1 with the winter solstice, but is believed to be because he was very superstitious wanted the January 1 would coincide with a new moon.
Sources: Microsiervos, El blog de Mosqueira