International Phelan-McDermid Syndrome Day, also known as International Phelan-McDermid Syndrome Awareness Day, is observed worldwide every October 22. Established in 2018, this day aims to highlight the rare genetic disorder known as Phelan-McDermid syndrome (PMS), which is characterized by global developmental delay and other neurological and behavioral symptoms due to deletions or rearrangements on chromosome 22. Common features of PMS include moderate to severe intellectual disability, absent or severely delayed speech, low muscle tone, and autism spectrum traits. The syndrome affects fewer than 2,500 people globally, making early detection challenging, particularly in areas with limited access to genetic testing. On this day, individuals and organizations are encouraged to learn about PMS, share information, and support affected families or research initiatives. Treatment typically involves a multidisciplinary team of specialists to manage symptoms and improve the quality of life for those impacted by the syndrome.