21 June to 22 July
Cancer opens at ninety degrees of ecliptic longitude, and that point is the June solstice itself: the Sun at its greatest northern declination, the longest day north of the equator and the shortest south of it. The season ends at a hundred and twenty degrees, on 22 or 23 July.
Both edges wander by a day. The Sun's circuit runs about a quarter of a day longer than the calendar accounts for, so each crossing arrives later for three years and is dragged back on the fourth, which is precisely what the leap day exists to correct.
A cardinal sign starting at a turning point
The four cardinal signs begin at the solstices and equinoxes, and Cancer's opening is the sharpest of them in one respect: it is a stationary point rather than a crossing. Declination stops changing for a moment and reverses, which is where the word solstice comes from.
The Moon rules the season and holds no other sign, the only ruler on the wheel with a single domain besides the Sun. Jupiter's exaltation falls at fifteen degrees of Cancer, in the first week of July, roughly at the middle of the range.
The sidereal answer is Gemini for most of it
Sidereal astrology keeps the zodiac attached to the constellations, while the Western zodiac is attached to the equinox. Those two anchors were together around two thousand years ago and have since drifted about twenty four degrees apart, because the Earth's axis traces a slow circle over some twenty six thousand years.
Adding that offset moves sidereal Cancer to roughly the middle of July through the middle of August. A tropical Cancer Sun below twenty four degrees is a sidereal Gemini Sun, and the two systems are answering different questions rather than contradicting each other.
The limits of a date
Near either edge the printed ranges are worthless, since they were rounded once and the boundaries have kept moving ever since. The year, the birth time and the birthplace resolve it exactly, and the outcome is a single sign.
Within the season the decans split the month between Venus, Mercury and the Moon in ten degree bands. A date cannot fix the Moon's own sign, which changes every two and a half days, and cannot touch the Ascendant, which depends on the hour and the latitude.
Questions people ask about this
What are the Cancer dates?
Approximately 21 June to 22 July. The exact start falls on 20 or 21 June and the end on 22 or 23 July, 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 Cancer 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 Cancer?
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 Cancer 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.