International Mastocytosis & Mast Cell Diseases Awareness Day is observed worldwide every October 20. It was established in 2018 by 24 mast cell advocacy groups from various countries, including the United States, Canada, and several European nations. The day focuses on increasing awareness of mast cell diseases, which can range from benign to life-threatening conditions. These diseases, such as mastocytosis, mast cell activation syndrome, and hereditary alpha tryptasemia, involve the inappropriate release of mediators by mast cells, leading to a variety of symptoms. Activities on this day aim to advocate for better diagnostics, access to treatment, and further research into these rare diseases.