Where the boundary actually is
The start of Aries is not a date somebody chose. It is the moment the Sun crosses the celestial equator moving north, which is what an equinox is, and in the tropical zodiac that crossing is defined as zero degrees of Aries. Every other sign boundary in the year is then measured off it in steps of thirty degrees.
Because the equinox is a physical event rather than a calendar convention, it does not fall at the same clock time every year. The Earth takes roughly 365.2422 days to complete an orbit, so each ordinary year the crossing slips about six hours later, and each leap year it jumps back by around eighteen. Watch it over a decade and you will see it wander across the 19th, the 20th and occasionally the 21st of March. The table above solves for the year rather than averaging across them.
How long the season lasts
Aries runs a little over thirty days, usually somewhere near 30.9. Signs are equal in space, thirty degrees each, but not equal in time, because Earth's orbit is an ellipse and the planet moves faster when it is nearer the Sun. Perihelion falls in the first week of January, so the winter signs are the short ones at about 29.5 days, and aphelion falls in early July, which is why Gemini and Cancer stretch to about 31.5.
Aries sits on the rising side of that curve, past the fast winter stretch and heading towards the slow summer one, which puts it near the middle of the range. The exact figure for your chosen year appears in the table, and it changes slightly from year to year because the boundaries at either end move independently.
The ruler, the element and what the month is taken to mean
Aries is a cardinal fire sign ruled by Mars. Cardinal signs open a season, fire signs act before they deliberate, and Mars supplies the appetite for going first. Put together, the traditional reading of the month is one of starting things: new projects, new arguments, new plans that have not yet met any resistance.
Whether that description does any work is a separate question, and this page does not settle it. What is checkable is the timing, which is what the table is for. It is also worth noting that the phrase Aries season, used as a way of naming the month rather than the sign, is recent. It came out of astrology on social media over roughly the last decade, where the signs became a way of labelling stretches of the calendar. Astrologers had always called this period the Sun's transit through Aries. The idea is old, the phrasing is not.
Cusp birthdays and the southern hemisphere
If you were born on 19 or 20 March, you were not born on a cusp, because there is no such thing. The Sun occupies one longitude at any instant and that longitude sits inside one sign. Born an hour before the equinox you are entirely Pisces, born an hour after it you are entirely Aries, and nothing about the two signs is mixed in between. Settling it needs your birth time and the time zone that was genuinely in force where you were born, which in many countries during the twentieth century was not the zone in force there now.
The dates are also identical everywhere on Earth, because the ingress happens at one moment for the whole planet and only the local clock reading differs. What changes below the equator is the season. In Sydney or Buenos Aires, Aries begins with the autumn equinox rather than the spring one, so the seasonal logic the sign was built on runs backwards even though the calendar does not.
Questions people ask about this
When does Aries season start?
On the equinox, which in recent decades has usually fallen on 20 March, occasionally on the 19th and rarely on the 21st. The precise moment for any given year is in the table above, given in UTC. Convert it to your own zone before deciding a borderline birthday.
Why is Aries the first sign if the year starts in January?
Because the zodiac is measured from the equinox, not from the civil calendar. The tropical system takes the March crossing as its origin, so Aries begins there by definition. January is a Roman administrative choice and has nothing to do with the sky.
Does Aries season always end on 19 April?
Usually, but not always. The end of Aries is the moment the Sun reaches thirty degrees of longitude and enters Taurus, and that moment drifts across the 19th and 20th of April in the same way the start drifts in March.
Is Aries the shortest sign?
No. That distinction belongs to the signs the Sun crosses closest to perihelion in early January, which currently means Capricorn and its neighbours at about 29.5 days. Aries is nearer thirty-one.
Is Aries season the same in Australia?
The dates and times are exactly the same, because the Sun's ingress is a single instant for the whole planet. The season is the opposite one, autumn rather than spring, which is a genuine difficulty for the seasonal reasoning behind the signs but does not alter the calendar at all.