Lost Monday (French lundi perdu or lundi parjuré) is a Belgian tradition that is generally celebrated on the Monday after the Sunday after Epiphany. This tradition has remained steadfast in the province of Antwerp and in Tournai. In Flanders and especially in the province of Antwerp, it is tradition to eat sausage breads and apple dumplings on that day.
In Aalst, Lost Monday is in early October. In Sint-Truiden, Lost Monday is the Monday before Ash Wednesday; then the annual carnival procession walks through the streets of the city.
Many stories and city legends are circulating about the origin of this custom, one more accurately dealing with historical information than the other. But one thing is sure, it's an Belgian tradition for everyone to enjoy.