Alice Liddell (the girl the ‘Alice’ stories were based on) was born on May 4. So today is the only day in the year she was not able to celebrate her Unbirthday. Happy birthday, Alice!
When you read the story closely, Alice will tell you on seperate occasions on which date the story takes place. First, she tells you it is May, later that it is the fourth. So she actually visits Wonderland on her birthday!