The solstice always occurs between 20 and 22 June, though during a leap year it generally falls on 20 June.
The exact date shifts slightly each year because the Earth takes almost a quarter of a day longer to complete its orbit of the Sun than our calendar accounts for.
This discrepancy is why we add a leap day every four years – to keep the calendar aligned with Earth’s orbit.
In English, the world solstice comes from the Latin word solstitium, meaning “sun standing still”.
It seems to suggest a brief pause as the Sun reaches its most extreme point.