Both edges of the season, and why they slide
Aries season opens on 20 or 21 March and closes on 19 or 20 April. Its opening is the March equinox, which makes it the only sign whose first day is also the origin of the entire measuring system: zero degrees of ecliptic longitude is defined as the Sun's position at that crossing.
The slide has one cause. The Sun needs about three hundred and sixty five and a quarter days to come back to the equator, and the calendar counts whole days, so the equinox arrives roughly six hours later each year until a leap day pulls it back. Nothing accumulates over the long run; the instant oscillates across a day and a half and returns.
The season measured against the year
Being first, Aries carries the four cardinal quarters with it: the Sun in Aries is the Sun in the first quarter of the tropical year, moving north and gaining declination for the whole month. Days lengthen fastest in the northern hemisphere during this stretch, which is a fact about the Earth's tilt and the reason the sign was placed here rather than a claim derived from the sign's character.
Mars rules the season, and the Sun's exaltation at nineteen degrees of Aries lands in the middle of April, near the end of the range rather than at its start.
Sidereal Aries begins three weeks later
Indian astrology measures the zodiac from the fixed stars rather than from the equinox, and the two frames have separated by roughly twenty four degrees since they last coincided. Precession moves the equinox point backwards through the stars in a cycle of about twenty six thousand years, and that separation is its accumulated total.
Twenty four degrees is a little over three weeks of solar motion, so sidereal Aries opens in the middle of April. Almost everyone with a tropical Sun in Aries has a sidereal Sun in Pisces. The two frames start from different reference points and neither is an error in the other.
Where the range stops being useful
Within roughly two days of either edge a published range cannot decide anything, because the ranges were rounded to whole dates and the ingresses have never fallen on whole dates. The birth year, the recorded time and the birthplace settle it against an ephemeris, and the outcome is one sign with nothing left over.
The three decans divide the month more finely than the sign does: Mars, then the Sun, then Venus over the ten degree bands. Beyond the Sun, a date alone fixes nothing — the Ascendant covers all twelve signs in any twenty four hours.
Questions people ask about this
What are the Aries dates?
Approximately 20 March to 19 April. The exact start falls on 20 or 21 March and the end on 19 or 20 April, varying with the year and with the time zone the birth is dated in.
Why do the dates change from year to year?
Because the tropical year is not a whole number of days. The ingress slips about six hours later each year and is pulled back by each leap day, so it oscillates across roughly a day and a half.
When is Aries season this year?
Within the range above, at a minute that has to be calculated for the specific year. Anyone born close to either edge needs the actual solar position rather than a table.
Why does Vedic astrology not make me Aries?
It measures from the fixed stars rather than from the equinox, and the two references have separated by about twenty four degrees. That moves every sidereal boundary later by a little over three weeks.
Does being born on the first day of Aries mean anything special?
It puts the Sun in the first degree of the sign, which matters for aspects and house placement. The tradition does not read it as a weaker or stronger version of the sign itself.