A stellium is made up of three or more planets in the same zodiac sign.
The more planets in the sign, the rarer the stellium.
Stelliums indicate a concentration of focus and energy in a particular sign, involving the themes of that sign.
During late February and March of 2013, we experienced a major stellium of bodies in Pisces. At its height, the stellium included six planets (the Sun, Moon, Mercury, Venus, Mars, Neptune) and Chiron, which is considered a comet nucleus.
Because Mercury and Venus travel close to the Sun in the zodiac, it is not uncommon to have those three bodies (Sun, Mercury, Venus) in the same sign in an astrological chart. There are 30 degrees in each sign of the zodiac. Mercury is never more than 28 degrees behind or ahead of the Sun. Venus is never more than 46 degrees behind or ahead of the Sun.
Mars takes a little under two years to move through the zodiac, and it goes retrograde about once every two years, as well. The largest stelliums generally occur when Mars has reached the area of the zodiac inhabited by the transiting Sun, Mercury, and Venus. This was the case with the recent Pisces stellium.
Large stelliums of five, six, seven, or more bodies are more rare.
In order to have a stellium of more than the five personal planets, one of the intermediary (Jupiter, Saturn) or outer (Uranus, Neptune, Pluto) planets has to also be in the sign.
The personal planets (Sun, Moon, Mercury, Venus, Mars) move fairly quickly through the signs of the zodiac. The intermediary planets (Jupiter and Saturn) move a bit more slowly through the signs. And the outer planets (Uranus, Neptune, and Pluto) spend the most time in each sign - approximately six to twenty years in each, depending on the body.
One or more of the slower-moving bodies must be inhabiting the sign in order for one of the largest stelliums to form.
According to my research, the next seven-planet stellium will occur September 5 and 6, 2051 in Virgo. The Sun, Moon, Mercury, Venus, Mars, Jupiter, and Uranus will be in Virgo at that time.
There will also be a seven-planet stellium in Aquarius on the last day of Aquarius season, February 18, 2080. This will actually be an eight-body stellium because dwarf planet Ceres will also be in Aquarius at that time, along with the Sun, Moon, Mercury, Venus, Jupiter, Saturn, and Uranus. Using Ceres, the seven-planet stellium is extended for about a week in February.
The most recent seven-planet stellium took place May 3, 2000 with seven planets in Taurus - the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
Prior to that, a stellium was experienced January 10 - 12, 1994 with seven planets in Capricorn - the Sun, Moon, Mercury, Venus, Mars, Uranus, and Neptune.
Prior to that, there was a seven-planet stellium in Aquarius February 18 - 20, 1962. That stellium involved the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, and the South Node.
The Aquarius stellium in 1962 has been linked to the official start of the astrological Aquarian era.
If that Aquarius stellium marked the official beginning of the Aquarian era, the Pisces stellium we just experienced can be seen as the official end of the astrological Piscean era. Up to this point, we have been living in a layered, transitional time between eras when characteristics of both were prevalent. Now, we tip into the stronger Aquarian overlay and find a new expression for Pisces for the astrological Aquarian era.
Six-planet stelliums are also notable but are less rare than seven-planet stelliums. Some of the six-planet stelliums coming up include:
A six-planet stellium in Capricorn in January 2018.
A six-planet stellium in Aquarius in February 2021.
A six-planet stellium in Aries in April 2026.
A six-planet stellium in Gemini in June 2028.
A six-planet stellium in Gemini in May 2032 - this does not involve the Moon, so it's substantial.
A six-planet stellium in Scorpio in November 2041.
A six-planet stellium in Taurus in April 2047.
This is not an exhaustive list, but it does give some perspective on large stelliums.
They are not necessarily once-in-a-lifetime astrological events, but they are certainly not common. The meaning ascribed to each stellium will vary based on astrologer.