The earliest possible date for the summer solstice in the Northern Hemisphere is June 20. In the Gregorian calendar, the June solstice generally occurs on June 20 or June 21, depending on the year and the time zone used.
The solstice is not an entire calendar day but a precise astronomical moment, when Earth’s North Pole is tilted maximally toward the Sun and the Sun reaches its greatest northern declination. Local clocks can therefore place the event on different dates near midnight, even though the underlying moment is the same.
Calendar conventions explain the variation. Earth’s seasonal cycle is not exactly 365 days, so leap years and the Gregorian calendar’s adjustments keep the solstice from drifting indefinitely. June 22 is not the normal modern Gregorian date for the Northern Hemisphere summer solstice, while June 23 is associated with older traditional observances in parts of Europe rather than the astronomical event itself.