The whole thing came about due to problems that arose during the Middle Ages: war, famine and disease. The authorities would often impose fasting on the populace, young and old, rich and poor alike, during these tough times. During these days, pilgrimages, processions and acts of penance would also take place and both Protestants and Catholics would take part. It wasn’t until 1832 that the Jeûne fédéral became an official day of Thanksgiving throughout Switzerland.
In Geneva, another similar day is celebrated in September and is called Jeûne genevois. Originally, the day was marked with a fast in October, supposedly for Genevans to show solidarity with their fellow Protestants in France, who were at that time being persecuted for their faith. However, after the St. Bartholomew’s Day massacre, when thousands of Protestants met an untimely end, a fast was organized on September 3rd to remember those who were slaughtered. Over time in Geneva, as with the rest of Switzerland, the day stuck and later, in around 1792, it was dusted off and made into a day of celebration once again and is still celebrated today as a cantonal, and immensely patriotic, holiday.
Nowadays, both the Jeûne fédéral and the Jeûne genevois have lost much of their religious veneer and many people probably won’t be able to tell you where the day comes from. As you might expect, the tradition of fasting often goes out the window and it is simply considered a national holiday and an excuse to take to the mountains or spend some time with family. In Geneva, the tradition is to eat plum pie. If you visit Switzerland during this period, don’t be surprised to see most shops closed and the streets eerily empty.