The day of a president's funeral has traditionally been a national day of mourning. Financial markets were closed, for example, Friday, June 11, 2004, after President Reagan died. After President Ford died on December 26, 2006, the markets were closed on January 2 of the following year, even though that meant they'd be dark for four straight days, including the New Year's holiday.
The NYSE has traditionally observed the death of former and sitting presidents by closing for an entire day, starting at the onset of the 20th century. Former President William McKinley's funeral was the first to be so honored on September 19, 1901.