when was the croatian parliament formed?
19 April 1273
Croatian Parliament
The oldest Sabor with extant records was held in Zagreb on 19 April 1273. 
wiki:19125856
Croatian Parliament [SEP] Introduction
The oldest Sabor with extant records was held in Zagreb on 19 April 1273. This was the Sabor of Slavonia, and not of Croatia and Dalmatia. The earliest Sabor of the Kingdom of Croatia and Dalmatia dates to 1351. The Parliament session held in 1527 in Cetin affirmed the House of Habsburg as Croatian rulers. After this, the Sabor became a regular gathering of the nobility, and its official title gradually stabilised by 1558 as the Parliament of the Kingdom of Croatia and Slavonia. Since 1681, it has been formally called the Diet of the Kingdom of Croatia, Dalmatia and Slavonia.