Finland, as a region, has a long history dating back over a millennia. The celebration that the Fins are celebrating is the centennial as Finland being an independent nation. In 1809, Finland was incorporated into Russia and remained under Russian rule throughout the 19th century. This lasted more than a century, when, during the Russian Revolution of 1917, Finland declared themselves independent of Russian rule. This declaration was pretty momentous, but not universally celebrated on all sides. This declaration also caused a chain of events that led to an internal civil war the following month between socialist and non-socialist Fins.
The Civil War had international complications, with Germany supporting the "Finnish Whites" and Russia supporting the "Finish Reds" with the intent for either side to back the new Finnish government that would emerge at the end of the war. While the civil war ended in May 1918, neither backing country was able to put their plan into effect due to Germany losing the First World War. Thus, Finland emerged as a democratic republic.