en.wikipedia.org/wiki/A_Monument_to_Something_That_Never_Happened

Tarnów city and tourism sources consistently place the unveiling/opening on 29 August 2010.

• The Tarnowskie Centrum Informacji monument guide lists this work under "Instalacja Wilhelma Sasnala w Tarnowie – Mościcach" and gives "Data odsłonięcia: 29 sierpnia 2010 r." (date of unveiling: 29 August 2010).
• The City of Tarnów's project announcement for "Tarnów. 1000 lat nowoczesności" gives the relevant program for Sunday, 29 August, including a walk to Wilhelm Sasnal's sculpture after the film screening in Mościce, indicating the public opening event was on that date.
• A public-art documentation page about the sculpture likewise states that "Praca 28.03.1983 została odsłonięta 29 sierpnia 2010 roku" (the work was unveiled on 29 August 2010).

Because these sources specifically identify 29 August 2010 as the unveiling/opening date, the article's statement that it was unveiled on 28 August 2010 is incorrect.

Official Tarnów sources place the unveiling/opening on 29 August 2010.

• Tarnowskie Centrum Informacji's monument guide lists Sasnal's Mościce installation with "Data odsłonięcia: 29 sierpnia 2010 r."
• The City of Tarnów's program for "Tarnów. 1000 lat nowoczesności" schedules the public event around the sculpture on Sunday, 29 August.
• A public-art documentation page for the work states that "Praca 28.03.1983 została odsłonięta 29 sierpnia 2010 roku".

So the page's infobox date of 28 August 2010 is incorrect by one day.