Short answers, pulled from the story.
Julius Caesar established January 1 as the official start of the year in 45 BC when his Julian calendar took effect. This date aligned with when civil officials assumed power and when the Senate convened annually.
Nowruz marks the Iranian New Year at the exact moment of the northward equinox occurring March 20 or 21 each year. Zoroastrian communities celebrate this same day alongside Parsis living in India and Persians globally.
Georgia, Israel, Russia, North Macedonia, Serbia, Montenegro, and Russian-occupied Ukraine mark civil new year on January 1 while religious feasts occur on January 14 Gregorian. Most countries today observe January 1 as the first day of the civil year under the Gregorian system.
The Islamic calendar counts only lunations ignoring solar seasons totaling about 354 days per year. Its New Year occurs eleven days earlier each year relative to the Gregorian calendar.
Scotland adopted Modern Style on the 1st of January 1600 by Order of the King's Privy Council. The Calendar Act passed in 1750 converted the British Empire to the Gregorian calendar and declared January 1 as the civil new year for England, Wales, Ireland, and colonies.