To explain things briefly, in 45 B.C., the Julian calendar designed by Emperor Julius Caesar of the Roman Empire was introduced with a leap year day inserted every four years. At first, according to several scholars, the years that were declared leap years occurred too tight in frequency, occurring every three instead of every four years.
Augustus Caesar introduced a correction where the calendar the Romans actually used (hence I call it the Julian Actual/Transitional version) in 9 B.C. so that, the following 12 years, from 8 B.C. through 4 A.D. would have no scheduled leap year, allowing the calendar to catch up with the intended design of the Julian calendar. When the leap years resumed every four years at 8 A.D., it was the Julian calendar the way Julius envisioned in 46 B.C.
As the decades wore on, as it turned out, the Julian calendar (going through design changes ranging from the days of the month naming to the year of reckoning to 1 B.C.), was ten days slow when the 16th century arrived, resulting in the actual solar equinoxes arriving nearer the beginning of March and September than the end of those months.
In 1582, Pope Gregory XIII introduced the Gregorian calendar, resetting the date so that October 4 would be followed by October 15.
Pope Gregory XIII introduced his Gregorian calendar to correct the timing of the days of months by using the century (the third) where the first Easter day was observed, and lining up the dates of the century between the Gregorian (proleptically) and the Julian-1 calendars.
Gregory's calendar skipped the years that were divisible by 100 but not by 400 as leap years, while every other year that was a multiple of four were leap years. Counting backwards from 1 B.C. inclusive and every four years, it was a leap year except in years following the sequence of 101 B.C., 201 B.C., 301 B.C. (401 B.C. was a leap year), 501 B.C. and so forth proleptically.
Pope Gregory XIII's calendar was adopted gradually and is now the defacto calendar of Earth.
As I discovered, when you go back proleptically on the Gregorian calendar, back to the years 5 B.C. through 2 B.C., there is a coincidental lineup that Gregory XIII might have overlooked, but an added benefit of his calendar reform.
When going back in time, you may notice that the dates of the Gregorian and Julian calendars get closer and closer until you get to the century, the 200-300 A.D. range, where the dates line up perfectly between March 1 and February 28. That was Gregory XIII's intentions of his design, which was, to line up the proleptic Gregorian dates to the Julian dates to the century where the first celebrations of Easter, the resurrection of Jesus Christ, occurred. Try as I might, I can't pinpoint the exact years or places they may have happened.
We continue down backwards until we reach 4 A.D., and at this point, the Julian (the corrected timing of the leap years according to Caesar), would become proleptic. It's because the correctional version of the Julian calendar I dub the Julian-Transitional was in use without the leap years from 8 B.C. through 4 A.D.
On March 1, 4 A.D., the Julian-Transitional and the Julian calendars lineed up date to date, with the Gregorian calendar stating it was February 28. The day before, February 27 Gregorian, it was February 29 on the Julian, but February 28 on the Julian-Transitional, which was the calendar the Romans actually used to remind you.
Going backwards four more years, on March 1, 1 B.C., Gregorian date, it was March 3 on the Julian and March 2 on the Julian-Transitional. The day before, February 29, Gregorian date, March 2 on the Julian and March 1 on the Julian-Transitional.
The day before that, February 28, Gregorian date, March 1 on the Julian and February 28 on the Julian-Transitional. On that day, the Julian-Transitional calendar had two more years that would have been leap years that needed to be skipped, thus there was no February 29, 1 B.C., 5 B.C. and 4 A.D. on the Julian-Transitional calendar. Also on that same day it was the last day of the fourth and final year that the calendars the Romans used to fix the leap year frequency problem lined up with the proleptic Gregorian calendars in the date range of March 1, 5 B.C. until February 28, 1 B.C.
For this theory, for the years that were leap years from 45 B.C. through 9 A.D., we'll use scholar Scalinger's theory on where the leap years on the Julian-Actual-1 calendar may and may not have happened.
According to scholar Scalinger, the leap years were all in B.C.: 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12 and 9. The first Julian Day, January 1, 45 B.C. Julian-Roman-Actual-1, mapped to January 2, 45 B.C. on the proleptic Julian-Roman-1 calendar. The first aligned day was on Feb 25 4 A.D. on the proleptic Julian-Roman-1 calendar. The Quadriennial leap year resumed on 8 A.D..
What I discovered is that when I get down to the 5-2 B.C. range, and I'm guessing that Pope Gregory XIII never reckoned when thinking of his Gregorian calendar, is that the March 25 date of the conception of Jesus Christ lines up between those same four dates between the Gregorian calendar and the Julian-Transitional-1 calendar. So for those who celebrate the Feast of the Annunciation, also known as the Immaculate Conception of Mary on March 25, if March 25 on the Julian-Transitional-1 calendar is the correct date of this (it maps to March 27 on the Julian-1 calendar between years 5 and 2 B.C.), it maps exactly to March 25 between those same four years! On the proleptic Julian-1 calendar, it's actually on March 27, but on the calendar the Romans used to correct the timing of the leap year error, it's March 25, the same as that on the proleptic Gregorian calendar! Could this be a Jesus miracle that Pope Gregory XIII never knew about? Also, it's the day celebrating God's preservation of the Blessed Virgin Mary from original sin.
So as you might conclude based on this theory, between the calendars the Romans used and the proleptic Gregorian calendars which would be invented in 1582, and let's throw in December 25 into this as well, the dates of the conception, birth and the first century's dates (3rd A.D.) celebrating the resurrection of Jesus Christ as Easter Day all line up perfectly with each other! While ignoring the intended timing of Caesar's Julian calendar, it was the dates the Romans used between 5 B.C. and 2 B.C. to stamp the events needed no date conversion to the Gregorian calendar.
For those keeping score, March 25 Gregorian/Julian-Transitional maps to March 27 Julian while December 25 Gregorian/Julian-Transitional maps to December 27 Julian.
So in this case, the people in charge of the timing of the leap years of the early days of the Julian calendar had no idea what was yet to come during the years of the timing of the leap year insertions were to be corrected and the invention of the Gregorian calendar 16 centuries later.
See this excerpt:
|Mar 25, 02 B.C.||Mar 27, 02 B.C.||Mar 25, 02 B.C.|
|Mar 25, 03 B.C.||Mar 27, 03 B.C.||Mar 25, 03 B.C.|
|Mar 25, 04 B.C.||Mar 27, 04 B.C.||Mar 25, 04 B.C.|
|Mar 25, 05 B.C.||Mar 27, 05 B.C.||Mar 25, 05 B.C.|
Also note that around 5 A.D. when the corrections of the leap year timing were completed after skipping the third year where the intercalary day should have been to speed the calendar up three days, the Julian-Roman-Actual-1 was caught up with the intended dates if the Julian-Roman-1 calendar, so the Julian-Roman-Transitional-1 period ended at that point.
For the full trail of the calendars used going from 2100 A.D. to 100 B.C., see Gregorian-Julian Differences By Century