Ostrów Lednicki is an island in the southern portion of Lake Lednica in Poland, located between the cities of Gniezno and Poznań. The word 'ostrów' means 'holm' (i.e., river or lake island) - hence in English it is sometimes known as 'Lednica Holm'.

A gród (a medieval Slavonic fortified settlement) was built here in the Middle Ages. Existing ruins of a chapel, church and palace, thought to be the home of the first Kings of the Piast dynasty, have been roofed over for preservation. Today the ruins are of archaeological significance, and the site of the Museum of the Piast Dynasty, opened in 1969. It is Poland's largest open-air museum.

The site is one of Poland's official national Historic Monuments (Pomnik historii), as one of the first such monuments designated on September 16, 1994. The list of national monuments is maintained by the National Heritage Board of Poland.

The island is important in the national history of Poland. During the reign of the first ruler of the Polish state, Mieszko I, and Boleslaw the Brave, it stood as one of the main defensive and administrative centers of the kingdom. The ruined castle and other buildings here were constructed during Mieszko I's reign, just before the year 966.

It's also one contender for the historical site of the personal baptism of Mieszko I, his wife Dobrawa of Bohemia, and his entire court, which took place on the Holy Saturday of April 14, 966. This date is the Baptism of Poland, the historical introduction of Christianity in Poland. The event also arguably marks the beginning of the Polish state.

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