Abstract: Instabilities and long term shifts in seasons, whether induced by natural drivers or human activities, pose great disruptive threats to ecological, agricultural, and social systems. Here, we propose, quantify, and explore two fundamental markers of seasonal variations: the Summer and Winter Teletherms—the on-average annual dates of the hottest and coldest days of the year. We analyse daily temperature extremes recorded at 1218 stations across the contiguous United States from 1853–2012 to obtain estimates of the Teletherms, and to characterize their spatial and temporal dynamics. We observe substantial regional variation with the Summer Teletherm falling up to 90 days after the Summer Solstice, and 50 days for the Winter Teletherm after the Winter Solstice, and that in many locations, the Teletherm is better described as one or more sequences of dates—the Teletherm Period. We show Teletherm temporal dynamics are substantive with clear and in some cases dramatic changes moving across broad regions, suggesting links to climate change. We also compare recorded daily temperature extremes with output from two weather models finding considerable though relatively unbiased error.