January effect

PrepNuggets

Calendar anomaly that  during the first five trading days of January, stock returns are significantly higher than they are the rest of the year.  This is an anomaly because in an efficient market, traders would exploit this by buying ahead, which pushes prices up, and selling at the end of it, pushing prices down.   In doing so, the profit opportunity is eliminated.

Synonyms:
Turn-of-the-year effect