Purpose: The purpose of this thesis is to investigate if abnormal patterns concerning the rates of return during specific weekdays and months are observable for the companies in the OMXS30 during the period 2003-2010. A special focus will be put on the Monday effect anomaly.

Background: Investors have a tendency to search for investment opportunities. If errors exist in the pricing of stocks it indicates that anomalies are present and that the stock market is inefficient. Investors then have the possibility to utilize the anomalies in order to receive above average returns.

Method: This study is using data of stock prices from Nasdaq OMX in the period of 2003-2010. The strength and existence of the Swedish stock market efficiency is measured through autocorrelation-, chi-square- and regression tests. Average monthly stock returns are calculated on daily-, monthly-, and yearly basis. The returns are compared in order to examine if day-of-the-week and turn-of-the-year anomalies exist.

Conclusion: No Monday effect is found in 2003-2010. However, positive Thursday- and positive Friday effects are detected. A negative turn-of-the-year effect as well as a positive April effect is found. The investment opportunities that could be utilized in 2003-2010 due to the specific anomalies in the period do not necessarily imply that the same anomalies can be expected on the OMXS30 in the future.