The year AD 2000 will be a notable one. Many people are asking the  questions, `Will 2000 be a  leap-year?' and `Will it be the start of the new millennium?' 

Leap-Years.

Leap-years were introduced into the calendar by Julius Caesar.  They  are  necessary as the length of the year  is not an integral number of days. The Julian calendar uses the fact  that  the length of the year is close to 365  and a quarter days. So a basic year with 365 days with an additional  extra  day every fourth year will give a  good approximation.

This calendar was used until the 16th century when the small discrepancy  between the approximate length of  the year, 365.25 days, and the true length, 365.24219 days, added up  to  several days. Pope Gregory  realised that this meant that the date of Easter would eventually  not  fall  in the spring but would become  closer and closer to Dec 25, Christmas.

In 1582 the Gregorian calendar was instituted. It changed the rule  for  determining whether a year should be  a leap-year by stating that century years should only be leap years  if  they  were divisible by 400. The effect  of this is to make the adopted average year-length 365.2425 days, an  approximation that will only amount to  one day's error after 4000 years.

Britain adopted the Gregorian calendar in 1752 when september 2nd  was  followed by September 14th.

We  still use the Gregorian calendar and so the year 2000, which is  divisible by  400, will be a leap-year.

Millennia.

A millennium is a period of 1000 years. The question of which year  is  the  first year of the millennium hinges  on the date of the first year AD. 

Unfortunately the sequence of years going from BC to AD does not  include  a  Year 0. The sequence of  years runs 3 BC, 2 BC, 1 BC, AD 1, AD 2, AD 3 etc. This means that  the  first   year of the first millennium  was 1 AD. The one thousandth year was AD 1000 and the first day of  the  second millennium was AD  1001. 

It is thus clear that the start of the new millennium will be 1 Jan  2001.

Celebrations.
The year AD 2000 will certainly be celebrated, as is natural for a  year  with  such a round number but,  accurately speaking, we will be celebrating the 2000th year or the  last  year  of the millennium, not the start of  the new millennium. Whether this will be an excuse for more celebrations in the following year will have to be seen! 

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