Imagine a whole world covered in ice and snow: ice on the mountains, ice capping the sea, ice tumbling off the cliffs, whole caves and hills made of ice, ice crunching under your feet, and ice crystals glittering on your eyelashes. Imagine that, and you have an idea of what it's like to be in Antarctica.
Because of the earth's tilt and orbit around the sun, the poles receive less energy and heat from the sun. This results in only two polar seasons—summer and winter.
In summer at the poles, the sun does not set, and in winter the sun does not rise. These dramatic seasonal variations control the ecosystem, and, as a result, Southern Ocean sea life faces challenging conditions. Ice forms in winter over much of the sea around Antarctica, and rapidly retreats in the spring when the ice melts. Summer is a season of long days and slightly warmer temperatures when phytoplankton can grow rapidly and produce food for the entire ecosystem. High winds blowing from the continent mix the seawater, bringing high levels of nutrients necessary for phytoplankton growth near the sea surface, where there is sunlight.
So we have only two seasons in Antarctica: cold and colder. Or, perhaps, light and dark. There are about four months of 24-hour daylight (that's "summer"), four months of 24-hour night ("winter"), and two months on either end where the sun is either coming or going. From a temperature standpoint, it's coldest from about May through October, with the coldest months being August and September, and warmest from November through April, with the warmest months being December and January.
By the third week of October, daylight is a 24-hour-a-day business. By mid-December, temperatures have stabilized to around -1° C, though some days can reach 2° C or more. Since the air is so dry, this can seem very warm. People can sometimes walk around on days like this in shorts and T-shirts. Wind is a major factor, though. If there's the slightest breeze, the jackets go on. Antarctica tends to be a windy place, and the wind often has more to do with how cold it seems than the actual temperature.
In this climate, it is advisable to take plenty of warm clothing and preferably to dress in layers, so that you can take off or put on a layer or two as the temperature changes.