Summer and Winter Solstice. The summer solstice occurs once a year in December when the Sun's track across the Australian sky reaches its highest point. It is the day that has the most daylight hours of any in the year. The summer solstice usually occurs on 21 December, but can occur between 20 and 22 December. The winter solstice is the day of the year that has the least daylight hours of any in the year and usually occurs on 21 June but can occur between 20 and 22 June.