What is point of this you asked? To wear a dress. Not a skirt or a tunic/dress over trousers, but a real dress. The point of it really is just to celebrate personal style (if dresses are among the things you like) and to make the average image of people in the street a bit more colourful, whimsical and/or elegant.
I'll make sure to wear a dress that day. I haven't decided which one yet (depends on the weather), but I'll let you know.