January 27 (International Holocaust Remembrance Day). Official commemoration with Auschwitz survivors and Israeli, Polish and other officials.
The first camp to be used (therefore called Stammlager, 'main camp' in German). It is in a far more complete state than Birkenau, but is also much smaller. The camp consists of former Polish military barracks, which were requisitioned by the Nazis in 1940. Near the entrance, you will see the wrought iron gate bearing the infamous and mocking camp slogan, Arbeit macht frei - "work sets you free." Inside some of them you will find information material, boards, photos and personal belongings to illustrate the life and cruelties of this camp. The only remaining gas chamber is here. As indicated in the chamber, it was reconstructed to its wartime layout after the war. Other sights include solitary confinement cells used as punishment, the death wall memorial where several thousands of people were shot by firing squad, and a reconstruction of the gallows used in 1947 to execute camp commandant Rudolf Höss, on the site of the camp's Gestapo office.
The entrance to Auschwitz I has a museum with a cinema where a 15-minute film is shown, shot by Ukrainian troops the day after the camp was liberated. It's too graphic for children (if indeed you bring them to Auschwitz-Birkenau at all), and costs 3.5 zł, included in the price of a guided tour. Showings between 11:00 and 17:00, in English on the hour and Polish on the half hour. Informative and disturbing. The bookstores and public conveniences are here. Consider buying a 5 zł guidebook or 5 zł map.
The second camp, built after Operation Barbarossa to accommodate the influx of captured Slavs from the Soviet Union, and is around 3 km from Auschwitz I. You can still see the entrance gate, the railway track and ramp and a single row of preserved barracks; the rest were destroyed. Many of the fences, chimneys and watchtowers are still standing, which illustrate the camp's bewildering size. You can also see the buildings where incoming prisoners were shaved and given their "new" clothing, the ruins of the four crematoria and gas chamber complexes, ponds where the ashes of thousands of people were dumped without ceremony, and a memorial site. Walking through the whole site may take several hours, and the sheer scale of the site may be harrowing to visitors. If visiting during cooler months, be aware that in the evening (which comes early during the winter months), the temperature drops very quickly across such a flat open space.