Complex sentence: In 1680, Knut's Day was moved to 13 January and became known as "tjugondag Knut" or "tjugondedag jul" (the "twentieth day of Knut/Christmas").
Split into simpler sentences: In 1680, Knut's Day was moved to 13 January. It then became known as "tjugondag Knut" or "tjugondedag jul" (the "twentieth day of Knut/Christmas").