It is Sunday, at least according to one study conducted in Germany, by Swedes. Could it be because there is much less to buy? Because the cities empty out? Because walking in nature is overrated? Because you are supposed to go to church or are supposed to spend more time with family? Indeed the effect is stronger for married people. For former East Germans there is not a significant Sunday effect.
When I lived in Germany, Sunday was my least favorite day, as it was in Wellington, New Zealand, although that is not the case living in the United States.
For the pointer I thank Bruce Bartlett.