[Verginius Rufus]

The assassination in Sarajevo took place on June 28, not July 28.

June 28 was the anniversary of the famous battle of Kosovo in 1389.

[Kaslin]

History.com says it was in late June 1914, when Archduke Ferdinand of Austria was assassinated. So June 28^th would be correct. Source

World War 1 did start on July 28 exactly four weeks later. Source