The synchronous display of the firefly Photinus carolinus Green in the Great Smoky Mountains National Park has been enjoyed by park visitors and studied by scientists and naturalists for the past 17 yr. A degree-day model is presented offering a means of more accurately predicting the display season of P. carolinus to facilitate advance planning required for researchers and the management of the thousands of nightly visitors who come to witness the peak firefly display. A modified growing degree-day (mGDD) model (base 50°F, starting date 1 March) (centigrade equivalent base 10°C, denoted mGDDC) provided the best fit to phenology data collected over a 15-yr period. The predictive model yielded the following values for P. carolinus phenology: male emergence, 838.6 mGDD (range, 776–922) /465.9 mGDDC (range, 431–512); “good” display, 992.5 mGDD (range, 931-1075)/551.4 mGDDC (range, 517–597); female emergence, 1068.8 mGDD (range, 956–1224)/ 593.8 mGDDC (range, 531–680); and peak male display, 1094.2 mGDD (range, 1020–1182)/607.9 mGDDC (range, 567–657). The accuracy and range limits of using calendar dates versus degree-day values are presented. Additional degree-day values are provided for 14 other local firefly and 1 phengodid species to broaden the applicability of using degree-days to aid in prediction of adult firefly seasons.