First my two cents about when the decade/century started.
You can pick any friggin’ day you want because no one counted the years from Year 1 until over 500 years after the fact. If you insist that the counting of the decades and centuries are tightly intertwined with the birth of Jesus, then the 21st century should have started in 1997 because Jesus’ birth is now thought by most scholars to have taken place in 4 BCE.
By the way, Scholars calculate Jesus’ birth at 4 BCE is because King Herod, according to Mathews, was involved in the slaying of the innocent which occurred at the time of Jesus’ birth. However, Herod inconveniently died in 4 BCE. You either have to admit the New Testament might have …uh… stretched the truth a bit, or maybe Dionysius simply miscalculated the date of the birth by a few years.
And, now for Mr. Quatro’s query about the new year…
The Romans switched from March as the first day of the year to January sometime around 150 BCE. (The year 153 BCE was the year 600 in the Roman calendar, so that might have been a reason for the change.) Consuls were elected at the beginning of the year, so changing the day the year began would change the date the Consuls were elected. The current calendar came from the reforms of Julius Caesar in 45 BCE. He lengthened the months to their current length, and replaced the leap month that occurred in the middle of February with a leap day that was also put in the middle of February (February 24th).
January is named after Janus, the god of transitions and beginnings. He was a god with two faces, one looking forward and one looking backward. That’s pretty much shows that the Romans considered January as the beginning of the year way before Christianity came about.
April Fools day probably came about from the Roman celebration of Hilaria which took place on March 25th. (The word “hilarity” comes from Hilaria). This was a joyous festival celebrating the spring, and what better way to experience joy than making a prank at the expense of another person?
The tradition of Hilaria continued in the Christian era as the Feast of Annunciation which also was on March 25th. In many places, this feast was considered the beginning of the religious New Year. In fact, Dionysius Exiguus himself declared March 25th the beginning of the New Year. However, the legal New Year was still January 1st. In many places, you’ll see dates between January and March 25th marked with both years such as “1023/24”. The use of March 25th as the beginning of the religious New Year continued until the 18th century in many places.
So, January 1st was the legal New Year since the time of the Roman Republic, but March 25th was also considered a religious new year too. April 1st never was considered the beginning of the new year in either way.
By the way, having multiple new years is not unusual. In Judaism, the first month of the calendar is Nisan which is the month Passover falls in. Thus, 1 Nisan is new years day and marks the birth of the Jewish nation. However the year is numbered from Tishrei which is the month Rosh Hashanah occurs in. Thus, 1 Tishrei is also a new years day and marks the birth of creation.