This paper analyses whether individuals are influenced by the day of the week when reporting subjective well-being. By using a large panel data set and controlling for observed and unobserved individual characteristics, we find a large day-of the-week effect. Overall, we find a 'blue' Sunday effect with the lowest level of subjective well-being. The day-of-the-week effect differs with certain socio-economic and demographic factors such as employment, marital status and age. The paper concludes with recommendations for future analyses of subjective well-being data and design of data collections.