This is actually kind of misleading, since nowadays it's only celebrated in Naantali where a celebrity gets thrown into the water. We don't commonly throw our family members into lakes... https://en.m.wikipedia.org/wiki/National_Sleepy_Head_Day
Glad no one around me practices this tradition, I'd be thrown in the lake/river/bathtub every year. And I sleep like a log so people could probably carry me to the nearest body of water and I wouldn't wake up before hitting the water.
"Grandpa, tell us the story about when you fought in the war that wiped out all morning people on Earth!" I can promise you this: the first person to try and bring that cute custom to America... there will be no survivors.
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https://en.m.wikipedia.org/wiki/National_Sleepy_Head_Day