"Born on a Blue Day" is a memoir by Daniel Tammet, an autistic savant who has extraordinary mathematical abilities and synesthesia, a condition where he experiences numbers as colors and shapes. The book details his life experiences, including his childhood, struggles with social interactions, and the unique way he perceives the world around him. In the memoir, Tammet shares his journey of self-discovery and acceptance, highlighting the challenges and triumphs he faced due to his condition. He also discusses his remarkable ability to perform complex calculations and learn languages quickly, providing insight into the mind of a savant.