The article is devoted to the subject of popular calendar anomalies. According to the theory of finance, if investors act rationally, the market can be considered efficient. In such a situation, achieving an above-average rate of return is impossible, as securities reflect all available information about them. However, on the basis of many studies and assumptions of behavioral economics, numerous exceptions to this rule have been discovered, which have been called market anomalies or stock anomalies. Such a deviation is the "January effect" and "January barometer" described in this work. The aim of the article is to investigate whether there is a deviation on the Warsaw Stock Exchange in 2015-2020 called the "January effect" and also whether the return rate in January can be a good prognosis for the rest of the year. In the results of the analysis, the occurrence of the title calendar effects in the studied sample was not unequivocally stated.