WHEN & WHY Springs Start

Today is the 1st day of spring in the Northern Hemisphere. Though no rules of gorgeous weather that's not too hot and not too cold, the Earth's position relative to the sun says it's wonder time for the birds to start chirping.
The first day of spring arrives on varying dates from March 19-21 in different years for two reasons:
1.Our year is not exactly an even number of days
2.Earth's slightly non circular orbit
3.Constantly changes our planet's orientation to the sun from year to year.
This year, spring starts Sunday, March 20, at 7:21 p.m. EDT (23:21 UTC). That's when the so-called vernal equinox occurs. Equinoxes which mark the onset of spring and autumn and solstices which mark when summer and winter begin are points in time and space that mark a transition in our planet's annual trip around the sun.
It works by: Earth's multiple motions spinning on its axis and orbiting the sun are behind everything from day and night to the changing seasons. The sun comes up each day because Earth rotates once on its axis every 24 hours or so. Seasons are a result of Earth being tilted 23.5 degrees on its spin axis coupled with the planet's 365-day orbit around the sun.
Imagine Earth as an apple sitting on one side of a table, with the stem being the North Pole. Tilt the apple 23.5 degrees so the stem points toward a candle the sun at the center of the table. That's summer for the top half of the apple.
Keep the stem pointing in the same direction but move the apple to the other side of the table: Now the stem points away from the candle, and it's winter on the top half of the fruit. The very top of the apple, representing the north polar region, is in total darkness 24 hours a day, during that season.
At winter solstice, the sun arcs low across the Northern Hemisphere sky for those of us below the Arctic Circle, and the stretch of daylight is at its shortest. By the time of the spring equinox, days have grown noticeably longer. At the summer solstice, the sun gets as high in our sky as it can go, yielding the longest day of the year in the Northern Hemisphere.
As long ago as the fourth century B.C., ancient peoples in the Americas understood enough of this that they could create giant calendars to interact with the cycle of sunlight. They built observatories of stone to mark the solstices and other times important for planting or harvesting crops. Shrines and even tombs were also designed with the sun in mind.