The long sign, and why
Every sign occupies exactly thirty degrees of the ecliptic. The Sun does not travel through them at a constant rate, because Earth's orbit is an ellipse rather than a circle. Kepler's second law says a planet sweeps equal areas in equal times, which in plain terms means it moves quickly when it is close to the Sun and slowly when it is far away.
The furthest point, aphelion, arrives in the first week of July. Gemini is the sign immediately before that, so the Sun is crawling by the time it crosses this arc. The result is a season of roughly 31.4 days, beaten only marginally by Cancer. Compare that with the signs around perihelion in early January, which take about 29.5 days, and the spread across the year is close to two full days. A fixed date range cannot show you any of this, which is one reason the days column in the table above is worth a look.
Ending at the solstice
Gemini closes at the instant the Sun reaches its greatest northern declination, the June solstice, which is also the moment Cancer begins. That gives Gemini a boundary that is genuinely observable: the day the Sun stops climbing higher at noon and turns back. The word solstice means the Sun standing still, which is what the change of direction looks like from the ground.
The date of that turn moves across the 20th and 21st of June, occasionally the 22nd in earlier centuries, for the same leap year reason every other boundary moves. The Earth's orbital period is not a whole number of days, so the calendar catches up in jumps and the astronomical events drift between them.
Mercury, air, and what the month is said to be for
Gemini is a mutable air sign ruled by Mercury. Mutable signs close a season and are read as adaptable rather than fixed in purpose, air signs deal in language and comparison, and Mercury is the planet of speech, trade and short journeys. The conventional reading of the month is talk: correspondence, gathering information, holding two views at once without committing to either.
The phrase Gemini season itself is a good illustration of how recent this way of speaking is. Astrologers have written about the Sun's transit through Gemini for two thousand years, but naming a month after a sign as a shorthand for a mood belongs to the last decade or so of astrology on social media. The underlying period is unchanged, only the label is new.
Birthdays near either edge
Late May and late June birthdays are the ones where the printed range most often gets it wrong. There is no Taurus-Gemini cusp and no Gemini-Cancer cusp, because at any instant the Sun sits at a single longitude inside a single sign. A cusp is only ever a symptom of having been given a date range instead of a time.
Resolving a borderline case takes three things: the date, the clock time of birth, and the time zone actually in force at that place on that day. Many countries changed their zones or their summer time rules during the twentieth century, and using the current rule for a birth in 1962 is the most common source of a wrong answer. Once those three are in hand the sun sign calculator returns a degree rather than a guess.
None of this differs by hemisphere. The ingress is a single instant worldwide, so a reader in Chile sees the same UTC times as a reader in Norway. What differs is that Gemini ends at the winter solstice there rather than the summer one.
Questions people ask about this
Why is Gemini season longer than a calendar month?
Because Earth is approaching aphelion, its furthest point from the Sun, which falls in early July. Orbital speed is at its lowest around then, so the Sun appears to move through the zodiac more slowly and takes about 31.4 days to cross Gemini's thirty degrees.
When does Gemini season end?
At the June solstice, which usually lands on 20 or 21 June. That same instant is the start of Cancer season. The table above gives the exact time in UTC for whichever year you have selected.
Are the Gemini dates 21 May to 21 June?
That is the usual approximation and it is right most years. Both boundaries move by up to a day across the leap year cycle, so the range is a summary rather than a rule, and it should not be trusted for a birthday on either edge.
Does Gemini season fall in summer?
It is the final month of spring in the northern hemisphere and ends exactly as summer begins. South of the equator the same dates are the last month of autumn, ending at the winter solstice.
Which sign is longer, Gemini or Cancer?
Cancer, by a small margin, since it contains the aphelion itself. Both run to roughly thirty-one and a half days and the precise figures shift slightly from year to year as the boundaries move independently.