Normally our calendar years are made up of 365 days, but every four years we add a day to make what is known as a ‘leap year’.
The Gregorian calendar, the most widely used across the globe, measures a year to be 365 days following the Earth's orbit around the Sun.
However, the Earth's orbit takes 365.24 days to complete its solar orbit and an extra day every four years is added to keep calendar seasons synchronised with solar seasons.
If we didn’t do this the seasons would shift around the calendar, and in 750 years’ time June would find itself in the middle of winter.
The modern calendar
The origins of the modern calendar as we know it date back to the Roman times.
Julius Caesar introduced the Julian calendar in 46 BC, which consists of 365 days with 366 in every fourth year.
However, in his calendar February had 30 days and August 29.
When Caesar Augustus became emperor he changed this so "his" month had the same amount as Caesar’s. He reduced February to 28 days to accommodate the amendment.
In 1582 the UK switched to the Gregorian calendar and worked out the Julian Calendar didn’t actually reflect the actual time the Earth takes to orbit the sun, or tropical year.
The Gregorian calendar follows three rules to determine which years are leap years: if the year can be divided by four it is a leap year, but if it can be evenly divided by 100 or by 400 then it is not.
Despite this complicated process the calendar is still not perfect, every 3,236 years it will be out by a day.
Comparably, the Julian calendar is currently lagging behind the Gregorian calendar by 13 days.
Leap day birthdays
Statistics expert Courtney Taylor, writing on website About.com, says around 0.07 per cent of the world’s population was born on a leap day, or 4.8 million people
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3/35 The history of tea in Britain
Google celebrates the 385th anniversary of tea in the UK
4/35 Autumnal equinox 2016
Google marks the start of fall
5/35 Paralympics 2016
Google marks the start of the Paralympic Games 2016
6/35 Nettie Stevens
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7/35 Father's Day 2016
Google celebrates Father's Day
8/35 Elizabeth Garrett Anderson
Google celebrates Elizabeth Garrett Anderson 180th birthday
9/35 Earth Day 2016
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10/35 Ravi Shankar
Google marks Pandit Ravi Shankar's 96th birthday
11/35 Olympic Games in 1896
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12/35 World Twenty20 final
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13/35 William Morris
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14/35 St Patrick's Day 2016
Googlle celebrates St Patrick's Day on 17 March
15/35 Caroline Herschel
Google marks Caroline Herschel's 266th birthday
16/35 Clara Rockmore
Google celebrates Clara Rockmore's 105th birthday
17/35 International Women's Day 2016
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18/35 St David's Day 2016
Google marks St David's Day
19/35 Leap Year 2016
Google celebrates Leap Day on 28 February 2
20/35 Lantern Festival 2016
Google celebrates the last day of the Chinese New Year celebrations with a doodle of the Lantern Festival
21/35 Stethoscope Inventor, René Laennec
Google celebrate's René Laennec's 235th birthday
22/35 Valentine's Day 2016
Google celebrates Valentine's Day with a romantic Doodle
23/35 Dmitri Mendeleev
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24/35 "The televisor" demonstartion
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25/35 Professor Scoville
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26/35 Sophie Taeuber-Arp
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27/35 Charles Perrault
Google celebrates author Charles Perrault's 388th birthday
28/35 Mountain of Butterflies discovery
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29/35 Winter Solstice 2015
Google celebrate the Winter Solstice
30/35 St Andrew's Day 2015
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31/35 41st anniversary of the discovery of 'Lucy'
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32/35 George Boole
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33/35 Halloween 2015
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34/35 Prague Astronomical Clock
Google celebrates the 605th anniversary of the Prague Astronomical Clock, one of the oldest functioning timepieces in the world
35/35 Autumnal Equinox 2015
Google marks the autumnal equinox on 23 September
Those born on a leap year are invited to join the Honour Society for Leap Year Babies, which currently has 11,000 members.
There are surprisingly few Leap Year traditions, but the custom of women proposing to men is the most famous.
The legend goes that in 5th Century Ireland, St Bridget complained to St Patrick about women having to wait too long for men to propose, to which St Patrick allowed women to propose on 29 February.
Although the more observant may note this could still result in a wait of nearly four years, the tradition has nonetheless gained popularity since the 19th Century.Reuse content