D23 is the Official Disney Fan Club chartered in 2009. The significance of the number 23 is a reference to the year 1923. 1923 was the year when Walt Disney arrived in Hollywood, California and founded the Walt Disney Company.
D24 is something entirely different. Tokyo Disney Sea has a D24 original brand apparel clothing line. The reference to 24 is for the person who wants to be stylish 24 hours a day.
Here’s a Jack Skellington Bag from the line priced at ¥ 2,900