Day and night will get equal time for a brief moment Monday as much of the world heads into fall. The autumnal equinox arrives Monday, marking the start of the fall season for the Northern Hemisphere and the spring in the Southern Hemisphere. In the Northern Hemisphere, sunlight will gradually diminish each day until the winter solstice on Dec. 21. The fall equinox is often connected with harvest festivals in the Northern Hemisphere. The Northern Hemisphere’s fall — or autumnal — equinox can land between Sept. 21 and 24, depending on the year.