Important Days & Dates Quiz: Gregorian Calendar, Leap Years, Solstices and Equinoxes

He issued the bull Inter gravissimas in February 1582 and the new calendar took effect that October, correcting the drift that had built up in the older system and tightening the rule for leap years.

Thursday, 4 October 1582 was followed immediately by Friday, 15 October 1582, so ten dates vanished while the sequence of weekdays continued unbroken; the correction was needed because the Julian year of 365.25 days was about eleven minutes too long and had pushed the spring equinox ten days out of…

Under the Gregorian rule a year divisible by four is a leap year, but a century year is a leap year only if it is also divisible by 400.

The Julian calendar, in use from 46 BC, assumed a year of exactly 365.25 days, which is about eleven minutes longer than the true tropical year of roughly 365.2422 days, so it gained a day about every 128 years and the equinox slid out of place.

A year repeats another year's calendar when the odd days between them add up to a multiple of seven and both years are of the same kind, ordinary or leap.

Move forward one year at a time, adding one odd day for an ordinary span and two when a 29 February falls inside it.

On this date, the June solstice, the Sun's rays fall vertically on the Tropic of Cancer at 23.5 degrees north, so the Northern Hemisphere receives its longest day and shortest night while the Southern Hemisphere has its shortest day.

Twice a year, about 21 March and about 23 September, the Sun's vertical rays fall on the Equator, the circle of illumination passes through both poles, and day and night are of equal length over the whole Earth, which is what the Latin root of the word equinox, meaning equal…

The Calendar Reform Committee, set up under the chairmanship of the physicist Meghnad Saha, recommended a unified national calendar, and the Saka calendar was adopted with effect from 22 March 1957, which was the first day of Chaitra in the Saka year 1879.

The Gregorian cycle of 400 years contains 97 leap years, so it holds 146,097 days, and dividing that by 400 gives exactly 365.2425 days for the mean year.

The Earth's orbit is a slight ellipse, and it reaches aphelion, its greatest distance of about 152 million kilometres, in early July, while perihelion, its least distance of about 147 million kilometres, comes in early January.