No, spring does not always fall on March 21. From last year and until 2102, spring arrives on March 20, this Friday. The reason is not political but scientific: it is all a question of tilting the Earth and our calendar not quite perfect.
What is an equinox?
First of all the equinox designates the date on which the day has exactly the same duration as the night. This phenomenon occurs twice a year: to go from summer to fall (September 23) and then from winter to spring (March 19, 20 or 21). From these dates, either the days get shorter (after the September equinox) or they get longer (after the March equinox). From this Friday morning, we gain four minutes of sunshine each day.
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Until then, the spring equinox most often fell on March 21 - but also twenty times on March 19 in the 19th century. During this century, with rare exceptions where it will fall on March 19, this scientific phenomenon will take place on March 20.
What is going on in the solar system?
At 2h49'36 '' Friday March 20, 2020 (French time), the center of the sun was exactly vertical to the equator. The axis of rotation of the Earth being inclined, it describes a cone during its revolution around the Sun. The apparent movement of the star in the sky therefore varies throughout the year, the Earth leaning towards the Sun for six months and then moving away from it for the following six months.
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Why is the date of the spring equinox changing?
To this cosmic phenomenon is added an element specific to our calendar: the Gregorian calendar adopted in the 16th century. This does not exactly correspond to the duration of a rotation of the Earth around the Sun, i.e. 365.2422 days since our calendar has 365 days.
To compensate for this slight difference, we have introduced leap years (this is the case in 2020); every four years, the month of February includes one more day. But this "trick" creates a new imbalance that brings down the spring equinox a day earlier or later than March 21.