There was a time before global trade became a fact when you could eat only all things local, and so lobsters were just an ordinary meal eaten in Sweden. So people consumed whatever they could until at some point the supply got scarce and to save the population of lobsters some restrictions had to be set. With these restrictions, for almost the whole 20th century people could eat lobsters only during late summer. Since Swedes have always adored seafood, August then meant a huge lobster consuming feast.
Nowadays, though you can buy and eat lobsters whenever you want, it's still a special experience to enjoy tradition and join the Kräftskiva crayfish party.