Complex sentence: The famous Open Air Plays of Szeged (first held in 1931) are one of the main attractions; which are held every summer celebrated as the Day of the city on May 21.
Split into simpler sentences: The famous Open Air Plays of Szeged (first held in 1931) are one of the main attractions; they are held every summer. Szeged celebrates the Day of the city on May 21.