For us in the southern hemisphere, it is called the Winter Solstice. It is an astronomical event where the Southern Hemisphere (South Pole) has the maximum tilt away from the sun (extra bonus what angle is this tilt - it's in the graphic ?). The winter solstice is the shortest day of the year, with the least amount of daylight and the longest night.
Compare this with the Summer Solstice, December 22, when we are most tilted towards the sun when we receive almost 14 hours of sunlight. Because of the sun's position, if you stand in direct sunlight at noon on the winter solstice, your shadow will be the tallest it can ever be in natural light.
The entire Arctic circle never sees sunset and the entire Antarctic circle never sees daylight.
Something to discuss as a topic with your students. What about other planets do they have seasons? Who came up with the concept of the earth revolving around the sun? How is time measured taking into account the solstices? Even in the humanities, culturally do we Fijians have a concept for seasons? Plenty to do with a simple topic such as this. Get the children to research this. Food for thought.