In PRL (Polish People's Republic) times, the date was moved to 22 July to honour the day the PKWN Manifesto (a document that basically asserted communist authority over Poland) was issued in 1944. In 1989 after communism fell, the holiday was restored to its original 11 November date. To mark the occasion every year in front of the Tomb of the Unknown Soldier on Pl. Piłsudskiego in Warsaw there’s the official Changing of the Guard with all the big wigs, president, prime minister and top brass in attendance. Outside of the capital, cities all over the country hold their own parades as well.
If you're in Poland on this day you'll find that though a national bank holiday, most restaurants and bars will still be open; museums and shops on the other hand will be closed. Marking the occasion in Katowice each year is a wreath laying ceremony at the monument to Wojciech Korfanty in front of the Silesian Parliament. An orchestra and choir will perform in front of the Christ the King Cathedral (10:00) which is then followed by a holy mass (10:30). A solemn ceremony (12:00) then takes place at the monument to Józef Piłsudki but also includes a military parade. It's all topped off with a display of military equipment (13:30), back where it all started, in Plac Sejmu Śląskiego.
Things get a bit more lively in Chorzów too, as a patriotic picnic is also organised at Plac Europejski in front of the Silesian Stadium, where military equipment will also be displayed for everyone to peruse (11:00-17:00).