Complex sentence: Celebrated on 30 April (the 29th if the 30th falls on a Sunday), "Koninginnedag" is Queen Beatrix's official birthday--she was born on 31 January; 30 April was the birthday of her mother and predecessor, Juliana.
Split into simpler sentences: Celebrated on 30 April (the 29th if the 30th falls on a Sunday), "Koninginnedag" is Queen Beatrix's official birthday. Though the Queen was born on 31 January, the holiday is observed on 30 April as it was the birthday of her mother and predecessor, Juliana.