Aug. 24 tops five years of sick-leave data as America's sickest day, but the real culprit isn't a virus; it's near-perfect late-summer weather. Average highs settle right around 80 degrees in Boston, Chicago, Detroit, Minneapolis and Pittsburgh as heat and humidity finally ease across the Northeast and Midwest. Pair that with the last stretch of summer break before Labor Day, and the forecast starts winning arguments with your to-do list.