Established in 1907, the Itasca Biological Station and Laboratories is a field station focused on conducting and promoting biological and ecological research. Part of this mission consists of long-term ecological monitoring efforts taken on by station staff. This dataset is one of such long-term efforts. Station staff tracked and recorded dates when lakes completely froze each winter (ice on, recorded when >99% of the lake was covered) as well as when they completely thawed each spring (ice off, recorded when >99% of the lake was open). Monitoring focused on Lake Itasca, with several other lakes in the region being added as various staff became associated with the Itasca Station.