Jimmy Carter, the 39th president of the United States, died on Dec. 29 at the age of 100. To honor the former chief executive, President Joe Biden has declared a National Day of Mourning on Jan. 9. Carter’s funeral will be held that day at Washington National Cathedral in the nation’s capital. Biden will deliver the eulogy. What Is a Day of Mourning? A Day of Mourning is not new; its observance, accompanied by a government shutdown, followed the assassination of President Abraham Lincoln in June 1865. The same observance has been held for most presidents who have died after leaving office over the last 75 years or so. In the wake of President John F. Kennedy’s assassination in 1963, President Lyndon Johnson ordered a day of mourning….