On the Gregorian calendar the Northward equinox can occur as early as 19 March or as late as 21 March. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from 20 March each year.
The March equinox may be taken to mark the beginning of spring and the end of winter in the Northern Hemisphere but marks the beginning of autumn and the end of summer in the Southern Hemisphere.
In astronomy, the March equinox is the zero point of sidereal time and, consequently, right ascension. It also serves as a reference for calendars and celebrations in many human cultures and religions.
The March equinox is one point in time commonly used to determine the length of the tropical year. The mean tropical year is the average of all the tropical years measured from every point along the Earth's orbit. When tropical year measurements from several successive years are compared, many slight variations are found which are due to a variety of phenomena, including nutation and the planetary perturbations from the Sun. The following table shows the small variations in timing over a long period of time; the mean Tropical year lasts 365 days, 5 hours, 48 minutes, 45 seconds.
Variation from average tropical year length
+ 2m 7s
- 2m 21s
The point where the Sun crosses the celestial equator northwards is called the First Point of Aries. However, due to the precession of the equinoxes, this point is no longer in the constellationAries, but rather in Pisces. By the year 2600 it will be in Aquarius. The Earth's axis causes the First Point of Aries to travel westwards across the sky at a rate of roughly one degree every 72 years. Based on the modern constellation boundaries, the northward equinox passed from Taurus into Aries in the year −1865 (1866 BC), passed into Pisces in the year −67 (68 BC), will pass into Aquarius in the year 2597, and will pass into Capricornus in the year 4312. It passed by (but not into) a 'corner' of Cetus at 0°10′ distance in the year 1489.
In its apparent motion on the day of an equinox, the Sun's disk crosses the Earth's horizon directly to the east at dawn—rising; and again, some 12 hours later, directly to the west at dusk—setting. The March equinox, like all equinoxes, is characterized by having an almost exactly equal amount of daylight and night across most latitudes on Earth.
Due to refraction of light rays in the Earth's atmosphere the Sun will be visible above the horizon even when its disc is completely below the limb of the Earth. Additionally, when seen from the Earth, the Sun is a bright disc in the sky and not just a point of light, thus sunrise and sunset can be said to start several minutes before the sun's geometric center even crosses the horizon, and extends equally long after. These conditions produce differentials of actual durations of light and darkness at various locations on Earth during an equinox. This is most notable at the more extreme latitudes, where the Sun may be seen to travel sideways considerably during the dawn and evening, drawing out the transition from day to night. At the north or south poles, the Sun appears to move steadily around the horizon, and just above the horizon, neither rising nor setting apart from a slight change in declination of about 0.39° per day as the equinox passes.
The Babylonian calendar began with the first full moon after the vernal equinox, the day after the Sumerian goddess Inanna's return from the underworld (later known as Ishtar), in the Akitu ceremony, with parades through the Ishtar Gate to the Eanna temple, and the ritual re-enactment of the marriage to Tammuz, or Sumerian Dummuzi.
The Indian national calendar starts the year on the day next to the vernal equinox on 22 March (21 March in leap years) with a 30-day month (31 days in leap years), then has 5 months of 31 days followed by 6 months of 30 days.
The Julian calendar reform lengthened seven months and replaced the intercalary month with an intercalary day to be added every four years to February. It was based on a length for the year of 365 days and 6 hours (365.25 d), while the mean tropical year is about 11 minutes and 15 seconds less than that. This had the effect of adding about three quarters of an hour every four years. The effect accumulated from inception in 45 BC until the 16th century, when the northern vernal equinox fell on 10 or 11 March
The date in 1452 was 11 March, 11:52 (Julian)  In 2547 it will be 20 March, 21:18 (Gregorian) and 3 March, 21:18 (Julian).
The Jewish Passover usually falls on the first full moon after the northern hemisphere vernal equinox, although occasionally (currently three times every 19 years) it will occur on the second full moon.
The Christian ChurchescalculateEaster as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is 21 March. The Eastern Orthodox Churches use the older Julian calendar, while the western churches use the Gregorian calendar, and the western full moons currently fall four, five or 34 days before the eastern ones. The result is that the two Easters generally fall on different days but they sometimes coincide. The earliest possible Easter date in any year is 22 March on each calendar. The latest possible Easter date in any year is 25 April.
The northward equinox marks the first day of various calendars including the Iranian calendar. The ancient Iranian new year's festival of Nowruz can be celebrated 20 March or 21 March. According to the ancient Persian mythology Jamshid, the mythological king of Persia, ascended to the throne on this day and each year this is commemorated with festivities for two weeks. These festivities recall the story of creation and the ancient cosmology of Iranian and Persian people. It is also a holiday celebrated in Azerbaijan, Afghanistan, Pakistan, Turkey, Zanzibar, Albania, and various countries of Central Asia, as well as among the Kurds. As well as being a Zoroastrian holiday, it is also a holy day for adherents of the Bahá'í Faith and the Nizari Ismaili Muslims. The Bahá'í Naw-rúz is calculated using astronomical tables—the new year always starts at the sunset preceding the vernal equinox calculated for Tehran.
According to the sidereal solar calendar, celebrations which originally coincided with the vernal equinox now take place throughout South Asia and parts of Southeast Asia on the day when the Sun enters the sidereal Aries, generally around 14 April.
In Manipur, this day marks the beginning of a new year of the Meitei lunar calendar and the day is celebrated as Cheiraoba by the Meiteis in Manipur. Since Meiteis use lunar calendar for its religious and social and religious festivals, the day varies from year to year and Cheiraoba is observed on the first day of Sajibu (the first month of Meitei lunar calendar).
In Andhra Pradesh, Telangana, Karnataka and Maharashtra people celebrate new year ugadi set by Satavahana on the first morning after the first new moon from the sidereal vernal equinox. Also the calculations of the great Indian mathematician Bhaskaracharya proclaim the Ugadi day as the beginning of the New Year, New month and New day.
In the Indian state of Odisha, this day is celebrated as the New Year around 14 April. It is known as 'Vishuva Sankranti' (meaning "equal" in Sanskrit). In Kerala though the new year is on Chingam 1, the beginning of sidereal zodiac Leo, sidereal vernal equinox is celebrated much more than new year as 'Vishu'.
In Annapolis, Maryland in the United States, boatyard employees and sailboat owners celebrate the spring equinox with the Burning Of The Socks festival. Traditionally, the boating community wears socks only during the winter. These are burned at the approach of warmer weather, which brings more customers and work to the area. Officially, nobody then wears socks until the next equinox.