There are many beliefs about the patterning of positive and negative mood over the course of the week. Support has been found for ‘Blue Monday’, ‘Thank God it's Friday’ and Weekdays versus Weekend effects, although in relatively small studies and often with student samples. Using telephone questionnaire data from a large national survey (N = 340,000), we examined day-of-week (DOW) effects on positive and negative moods. Unlike prior studies, we also tested the potential moderating effects of four demographic variables on DOW. Strong support was found for better mood on weekends and Fridays, but there was minimal support for a Blue Monday effect and no differences were observed between Saturdays and Sundays. Demographics moderated some DOW effects: DOW effects were diminished for older and retired respondents, but there was little DOW difference by gender or presence of a partner. DOW is associated with mood, but not always in ways we believe.