The study presents the dates and duration of thermal winter period identified with the use of two methods, i.e. using monthly average, and mean daily temperature values. Data were obtained from 36 IMGW-PIB stations and covered the period from October to April, 1960/61-2014/15. It was shown that the date of the beginning of thermal winter, identified using monthly average air temperature values, occurred approximately 3 weeks later, and the date of ending 3 weeks earlier than the dates of the first and the last winter season. As a result, the duration of thermal winter, in classical terms, is shorter by as much as a month than the potential period in which winter days could be recorded. Both adopted criteria, i.e. monthly average and daily mean, showed very high variability of thermal conditions in the cold half-year. In the characterised 55-year period, thermal winter was recorded each year at only two stations – in Suwałki and Białystok. Almost in the entire country, there were thermal winters intermittent with warm periods. However, in a greater part of the country, the frequency was not higher than 10% and the occurrence was more frequent in the central and south-western parts of Poland. Variability of the beginning and ending dates in the analysed period did not show statistically significant changes. A slight tendency for shortening of the winter period (19 stations) and a reduction of the number of winter days (11 stations) was found.