21 May to 20 June, edges included
Sixty degrees of ecliptic longitude opens the season and ninety degrees closes it. In date terms that is 20 or 21 May to 20 or 21 June, and the closing figure is the June solstice, so Gemini is one of the signs whose season ends on an astronomical event that almanacs print independently of astrology.
The one day uncertainty at each edge comes from the mismatch between a tropical year and a calendar year of whole days. Roughly six hours a year of drift, cancelled by each leap day, keeps both ingresses moving inside a window a little wider than a day.
The last stretch before the Sun turns
Gemini is mutable, and the mutable signs sit at the ends of the four seasons where one quarter gives way to the next. The Sun spends this month completing its climb north, slowing as it approaches the greatest declination it will reach, and the days lengthen by less and less as the season goes on.
Mercury rules it, and rules Virgo as well, so the ruler of these dates is shared with a sign in the opposite quarter of the year. Gemini itself holds no traditional exaltation, which distinguishes it from most of the wheel.
Why Indian astrology moves the range
Two zodiacs are in use. The Western one is fastened to the equinox; the one used across India is fastened to the stars, and the equinox has slipped backwards against them by something near twenty four degrees over the past two millennia. The cause is precession, a slow conical wobble of the Earth's axis with a period of about twenty six thousand years.
Sidereal Gemini consequently runs from around the middle of June into the middle of July, and a birth in the first three weeks of the tropical range comes back as Taurus in that system.
What a date range leaves undecided
Anyone born within about forty eight hours of either edge should treat the range as unusable and calculate the actual position from the year, the clock time and the town of birth. The result is one sign; the tradition defines no shared or partial placement between two of them.
Inside the range, the decans give three groups rather than one: Jupiter over the first ten degrees, Mars over the second, the Sun over the third. Everything else a chart contains needs a time and a place, not a date.
Questions people ask about this
What are the Gemini dates?
Approximately 21 May to 20 June. The exact start falls on 20 or 21 May and the end on 20 or 21 June, 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 Gemini 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 Gemini?
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 Gemini 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.