What was "Black Sunday" as noted in the nonfiction book, The Worst Hard Time?

April 14, 1935, known as Black Sunday, began as a surprisingly bright, clear day in the south High Plains. People felt their prayers had been answered at last. But the mother of all dust storms was brewing in the northern plains and when it met with a cold front from Canada it formed the biggest dust storm of the entire Dust Bowl disaster. People learned quickly that their respite from the dust storms was just a brief one. Nature was toying with them.