From AMS Glossary
In the United States, a period of mild weather popularly supposed to recur each year, in later January; most pronounced in the Northeast and, to a lesser extent, the Midwest.
The daily temperature averages at Boston, computed for the years 1873 to 1952, show a well- marked peak on 20–23 January; the same peak occurs in the daily temperatures of Washington, D.C., and New York City. Statistical tests show a high probability that it is a real singularity. The January thaw is associated with the frequent occurrence on the above-mentioned dates of southerly winds on the back side of an anticyclone off the southeastern United States.