We just ended eight days of continuously below freezing weather in Boulder. It was the longest cold snap since 1983 – which was caused by the eruption of El Chichón.
The longest stretch within a calendar year below freezing in the lower 48 states was at Hermit, Colorado during the Ice Age Scare in 1975. Every day until sometime in at least April was below freezing. Unfortunately there is no data from April, so I don’t know exactly how long the streak lasted.