I've had a lot of time to think these days, and I am quite sure that I have stumbled upon a... breakthrough, in the field of social behaviors and history.
My theory is concerning Poya Days, or full moon days when we Sri lankans all get a holiday, and why we have them. Well, here it is :
Many thousands of years ago, back when Buddhism was just arriving, there lived a clan of werewolves in Sri Lanka. There were also regular humans around, as you might have been taught.
Now, these two "factions" could have lived in harmony if not for the fact that werewolves love kurakkan. And the villagers had it in abundance.
Also, the werewolves were at their most powerful during the full moon. Therefore, to protect themselves from the werewolf menace, the villagers built mini-forts and holed up in these dome shaped structures(Because everyone knows werewolves hate domes) and waited out the full moon with their kurakkan safely stored in the forts.
And so eventually the werewolves died of starvation since they couldn't get any kurakkan.
That is, in it's entirety, my theory on why we have poya days.
Below is the evidence -
Notice the rising sun, commonly seen in sunny paradise isles like ours. Also the distinctly tropical trees in the background.
And the final piece of evidence -