August 15th was a Polish national holiday to commerorate Polish Armed Forces Day and the Christian religion's Feast of Assumption
Armed Forces Day, previously known as Soldiers' Day, was marked in pre-war Poland on 15 August, in remembrance of the repulse of the Red Army from Warsaw in the so-called 'Miracle on the Vistula' of 1920.
When a communist regime was installed in Poland following the Second World War, the new authorities switched the celebrations (under a new name) to 12 October, owing to the impossibility of honouring a defeat of a Soviet army.
It was not until 1992, following the collapse of communism, that the original date of 15 August was reinstated.
Meanwhile, the Feast of the Assumption of the Blessed Virgin Mary, which has been celebrated by Catholics for centuries, has a distinctive dimension in Poland, with the culmination of a time-honoured pilgrimage.
All the big shops were closed but the city centre had lots of tourists and crowds, so the cafes and bars there were probably open.