World Castleman Disease Day, also known as World CD Day, is observed worldwide every July 23 to raise awareness of Castleman disease, a group of rare disorders characterized by enlarged lymph nodes. The day aims to support those living with the disease, honor those who have died from it, and promote further research. The Castleman Disease Collaborative Network (CDCN) plays a key role in this observance by raising funds to accelerate research, encouraging donations that reflect the date of the awareness day. Participants often wear blue, the color of Castleman disease awareness, and use social media hashtags like #WorldCastlemanDiseaseDay and #WorldCDDay to foster conversations about the condition. Castleman disease includes two main types: unicentric Castleman disease (UCD) and multicentric Castleman disease (MCD), with specific treatment options for each.