Not exactly on June 21st Summer solstice occurs, but around June 21st it does. ‘Around’ here means that one day before or one day after it may occur. Summer solstice occurs twice a year once in Northern hemisphere in June and once in southern hemisphere in December.
Depending on the location and time zone the date of Summer solstice varies. Summer solstice in Northern hemisphere may occur between June 20 to 22, while in Southern Hemisphere the date may be between Dec 20 and Dec 23.