This holiday is always celebrated on 29 August.
It commemorates 29 August 1944, which was the start of an uprising against the pro-Nazi government of the time.
In 1938, the Munich agreement established the independent Slovak state, led by Jozef Tiso. In June 1944 Tiso declared martial law allowing the German army to occupy the country on 29 August.
Up to that point, there had been limited and sporadic guerilla resistance operations across Slovakia. To counteract the planned occupation, a formal military plan was put in action by the exiled Czechoslovak government, Slovak partisans and deserters from the Slovak army. Headquartered in central Slovakia, the forces of the uprising consisted of several armoured units as well as part of the Slovak Air Force.
Despite some limited support from the allies, the uprising was crushed by the German forces before the end of October 1944. After the defeat, the remaining forces of the uprising continued with small guerilla actions until the end of the war.
This day was chosen in 2003 to be a national holiday for Slovakia as it represents a time when all Slovaks came together against an oppressive dictatorship.
However, some see this antifascist celebration as having a strong communist aspect and it isn't celebrated by all Slovaks.
On 29 August each year, wreaths are placed on a memorial to the Slovak National Uprising in Bratislava.