My St Andrew's Daily Missal tells us:
St Hyacinth, called the apostle of the North, was born in 1158 at the castle of Kamin, near Breslau. Having gone to Rome, he was received there into the Order of Friar Preachers by its founder, St Dominic, in the church of St Sabina. At the age of 33, he was made superior of the mission which this saint sent in Poland. St Hyacinth then went over to Austria, Poland, Denmark, Scotland and Livonia, everywhere preaching the word of God, which his numerous miracles confirmed. He died on the feast of the Assumption in 1257.
You can find out more at New Advent here.