National Sourdough Bread Day is observed worldwide every April 1, celebrating the traditional art of sourdough baking. Sourdough bread is considered one of the oldest types of leavened bread, with roots tracing back to the Sumerians around 6000 BC. The bread is leavened using a sourdough starter, a mixture of flour and water that contains wild lactobacilli and yeast, which gives it a distinctive mildly sour flavor. Historically, European peasants would bake sourdough bread weekly, saving a piece of dough to use as a starter for the next batch. The techniques for sourdough baking were brought to the New World by French bakers during the California Gold Rush and spread further during the Klondike Gold Rush. Celebrants often make their own sourdough starters, visit locations like San Francisco known for its sourdough, or purchase loaves from local bakeries. The day highlights the ongoing enthusiasm for sourdough among artisan bakers and baking enthusiasts who share tips and starters online.