Complex sentence: Construction began in, and Millennium Park was opened in a ceremony on, 2004, four years behind schedule; the three-day opening celebrations were attended by some 300,000 people and included an inaugural concert by the Grant Park Orchestra and Chorus.
Split into simpler sentences: Construction began in, and Millennium Park was opened in a ceremony on, 2004, four years behind schedule. The three-day opening celebrations were attended by some 300,000 people and included an inaugural concert by the Grant Park Orchestra and Chorus.