Apparently, it's Trading Cards for Adults Day. No, I don’t know who comes up with these things either.
The first cards I remember collecting appeared in shops in 1966. These were The Riddler's Riddle, featuring scenes from the Batman TV series and movie. A delightful set of 38 cards, each with—as the name suggests—a riddle to solve on the back.
One of my earliest memories is of opening a packet and discovering the cards inside were in Dutch.