According to the Wirri Guild of Aarau, Bachfischet is the oldest Swiss custom that has been continually celebrated since its beginnings. The earliest written reference to the event is in the minutes of the Aarau Council meeting on 3 September 1526. For centuries the stream was a lifeline for the city, representing a source of drinking water, industrial water and water for firefighting. It was therefore essential to clean it and remove the accumulated silt. Nowadays Bachfischet takes place every September. The stream is cleaned in line with traditional procedure, which takes about four days. The end is marked on the penultimate Friday before the autumn holidays with a procession of schoolchildren carrying their own hand-made lanterns through the dark old town of Aarau. The procession welcomes the return of the stream, and has been made annually since the nineteenth century. The children in the procession sing the Bachfischet song and are rewarded afterwards with free food and drink. A firework display rounds off the festival, with a traditional final bang bringing the event to an official close.
The name «Bachfischet», meaning «stream fishing», dates back to when fishing in the stream was permitted prior to cleaning. Teenagers amused themselves by splashing around in the bed of the stream. Children play in the stream today too, enjoying the natural and exciting playground it provides. The stream is the centre of many stories and events, demonstrating the great importance it has for the people of Aarau.