The June solstice last occurred on June 22 in 1975.
A solstice is an exact astronomical instant, not an entire day. The June solstice occurs when the Sun reaches its greatest northern declination, near the Tropic of Cancer. Depending on the year and the time zone used, the calendar date can be June 20, June 21, or—very rarely—June 22.
In 1975, the solstice fell on June 22 in Universal Time, making that year the most recent June 22 occurrence. The next June 22 solstice in UTC is expected in 2203. Local calendar dates can differ when the instant occurs near midnight, so a source should ideally specify its time standard.
The date shifts because the tropical year is about 365.2422 days, not exactly 365. Leap years periodically reset the calendar relationship. This is why the solstice date follows a gradual, repeating pattern rather than staying fixed on June 21.