Gustav's Day

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Gustav's Day (Kustaanpäivä) as the day is called after Gustav Vasa who reigned from the 6th of June in 1523 until 1560, when he died at the old age of 64. The day is Sweden's national day. 
My grandfather on my maternal side was born on 6th of June. For a long time his birthday had been marked the 7th and so he once he already was of age asked his mother, how it is, assuming that she must know. His mother assured that "Gustav's Day, that's when it was".