Basically, because zero was not known in Europe at the time when our calendar was being devised in Rome in the late 5th century by the Greek-speaking monk Dionysius Exiguus from Scythia (in present day southern Russia) the start of the A.D. era was set at the year 1, immediately preceded by the year 1 B.C. (that's like having -1 immediately followed by +1 on the number axis!). It would not make sense to include year 1 B.C. (before Christ) into the Christian era, and so the 1st millennium (the first 1000 years of the Christian A.D. era) must extend from the beginning of year 1 till the end of year 1000, then the 2nd millennium starts with 1001 and ends with 2000, the 3rd millennium goes from 2001 to 3000, etc.
One can find some more interesting details on this topic e.g. in this three year old article.
Why to celebrate years like 1999/2000/2001 anyway? There is nothing special about them in the general flow of time! Only the labels we happen to give them because of the arbitrarily chosen beginning of our calendar and of our use of the decimal number system seem to stand out.
If interested in uniquely looking dates, have you noticed that we have recently had the last, for quite some time, date composed of only odd digits? It was 19/11/1999 (the format is day/month/year). The next such odd-only date will be 1/1/3111. Instead we will soon have an even-only date again, 2/2/2000, the first such date since 28/8/888. (Noted by Julie Novotná in the 11/12/1999 issue of the Czech-Canadian bi-weekly Nový Domov).
mk, Jan. 9, 2000
By saying above "because zero was not known in Europe when our calender was established" I mean that we do not have to quarrel whether our era starts at the beginning of year 0 or year 1. It must have started at 0 hours on Jan. 1 of the year 1 A.D. (which is the same time instant as 24 hours of Dec. 31 of the year 1 B.C.). I thought that nobody disputes that. From that time instant, only 1999 years elapsed at 0 hours on Jan. 1 of the year 2000. So how could rationally thinking people celebrate the beginning of a new millennium a year ago?
I do not advocate to start counting things in everyday life from 0. Of course, to have year 0 is convenient to be able to calculate more easily the number of years elapsed between a B.C. and an A.D. date. This is what modern astronomers routinely do: in their calendar, year 1 B.C. is labeled year 0, year 2 B.C. is year -1, year 3 B.C. is -2, etc. But this does not imply, that we should retroactively change to beginning of our era to the beginning of year 0. This has nothing to do with any doubts when exactly (if at all) Christ was born. (If our era starts at the presumed birth of Christ, why do we celebrate Christ's birthday on Dec. 25 instead of on Dec. 31 or Jan. 1? Is this the original cause of the confusion about when a new millenium starts.)
It is an answer to the following quiz question:
Entering Third Millennium I recall one of the greatest surprises of my life when, a year ago, I learned about thousands grown up and apparently educated people quarelling about Millennium starting in 2000 or 2001. As we here seem to know our arithmetics, you may ask, why I mention this folly. Well, because there is one little quiz left, which I find rather cute:
While Millennium obviously starts in 2001, the Pope has declared 2000 as the start of the Catholic Millennium. Why???
It is difficult to believe that, advised by all cunning Jesuits, he could make such a blunder and one is inclined to look for some reasonable explanation. And there is one, exact and rigorous, which goes to the roots of our civilization and throws some new light on its contradictions. But ripe as I am, I'll leave it here and let you find out by yourself.
Have fun, Georges Metanomski
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The Answer:
Let me try to answer taking for granted all Christian traditions and believes.
To start with, Christ is the only person about whom one may legitimately say that he was born 7 days before his own birth. Indeed, He was born on the Dec. 25 of the year -1, or the first year BEFORE CHRISTUM NATUM.
The calendar situating 'CHRISTUM NATUM' 7 days after His birth has been created by Rome.
Why?
Simply because at the time being Christianity had no problems with Christ being a Jew, whose physical birth did not count compared with the spiritual birth, the brit mila, the circumcision. And January 1 became the highest Christian holiday: 'Seigneur's Circomcision', the obvious milestone for starting the Christian calendar.
Only later, when Rome became antisemitic, Christum Natum coinciding with Seigneur's Circumcision became a pain in the neck and a scandal. It was too late to back up, but at least one could avoid to rub in. January 1 became gradually a pure arithmetical device and the big show has been transferred to December 25 of year 1 BEFORE CHRISTUM NATUM.
The past century witnessed on the one hand Rome's complicity with Nazi crimes and, on the other hand some sincere trials to humanize the faith and to refuse fanaticism and intolerance.
The decision to create a new religious calendar illustrates clearly Rome's refusal of these humanistic ideas in favour of antisemitism, fanaticism and intolerance.
And, as Rome is one of the most important mass manipulators, her attitude should interest people concerned with human society and with its virtual improvements such as DD (Direct Democracy).
Daniel Leider, March 19, 2001
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A few more paragraph added to the above text in
a "special 'Merry Christmas 2004'" greeting:
...
On the other hand, the whole world, whether Christian or
not, whether religious or laic, whether criminal or
honest, counts their dates with respect to Christum Natum,
which is the greatest marketing success ever achieved by
any organization, so why assail it, why cut the branch on
which one is sitting?
On the second thought one sees the light. Millennium gave a pretext to finish once for all with the scandal of Christ's Jewishness and of Catholic chronology and holiness reposing on a Jewish Mitzvah.
Let me call the Catholic and Usual CHRISTUM NATUM respectively "CCN" and "UCN". We see that declaring the Catholic Millennium at 2000 defines Year 1 of CCN as -1 of UCN and transfers Catholics to live currently in 2004 CCN. UCN is of course kept as the flourishing marketing context, but Catholic summit of holiness quits the Jewish Mitzvah and moves to Beitlehem creche.
It's not by chance that 2005, sorry! 2004, saw first exorcists graduate from the Pontifical Regina Apostolorum University of Vatican and a nun crucified to death by an exorcist for having talked to him "arrogantly", which could only be devil's doing.
Georges Metanomski, Dec. 22, 2005