January effect

The January effect (sometimes called "year-end effect") is a calendar effect wherein stocks, especially small-cap stocks, have historically tended to rise markedly in price during the period starting on the last day of December and ending on the fifth trading day of January. This effect is owed to year-end selling to create tax losses, recognize capital gains, effect portfolio window dressing, or raise holiday cash. Because such selling depresses the stocks but has nothing to do with their fundamental worth, bargain hunters quickly buy in, causing the January rally. The Incredible January Effect by  Robert Haugen in an authoritative text describing the January effect.
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Refers to the historical of rising stock prices in the first few days of January. Studies have suggested this only holds for small capitalization stocks. In recent years, there is aiminised evidence of a January effect.