Why don’t we like Mondays?

Because you’re focused about how you predict you’ll feel, not how you actually feel in the moment:

An Internet survey revealed that day-of-the-week (DOW) stereotypes (i.e., “Monday blues”, “Wednesday hump day”, “TGIF”, etc.) were pronounced when subjects predicted their moods for each day of the upcoming week, less obvious when they remembered their moods from each day of the preceding week, and least apparent in the momentary moods they actually experienced on each day. In a second study involving 2-hour, in-home interviews, subjects reporting looking forward to weekends because of the lack of structure and discipline and the freedom to choose activities, yet much of their weekend time was spent fulfilling a need to be productive, which often involved activities that, in many ways, simulated paid work. The content of these interviews suggested that people tend to overvalue future discretionary time on the weekend, assuming that two days of uninterrupted idle time will be more enjoyable than it actually is. In a more general sense, these results suggest that the predicted and remembered moods in the initial survey were driven by DOW stereotypes, which facilitated rapid judgements given the simple scaled response format. However, the depth interviews revealed that moods experienced throughout the week are more nuanced in terms of how they are remembered, described, and linked to recently passed, current, and immediately anticipated events, perhaps explaining why DOW stereotypes were less obvious in the reported momentary moods.

Source: “(Tell me why) I don’t like Mondays: Does an overvaluation of future discretionary time underlie reported weekly mood cycles?” from “Cognition & Emotion”