It often happens that all the mayflies in a population mature at once ( the hatch ) , and for a day or two in the Spring or Autumn ( Fall ) , mayflies will be everywhere , dancing around each other in large groups , or resting on every available surface . It often happens that all the mayflies in a population mature at once ( the hatch ) , and for a day or two in the spring or fall , mayflies will be everywhere , dancing around each other in large groups , or resting on every available surface .