If I had to guess, I would say that the hotter the temperature (up to a point) the happier people are. It would make sense that relatively warm summer days would be associated with the most happiness. However, the research tells a different story…
One study conducted a daily survey of happiness levels of 75 participants over 17 months. Meanwhile, the researchers tracked daily temperature, wind speed, humidity, precipitation and hours of sunshine. Somewhat surprisingly, the researchers found that happiness is maximized at 57 degrees Fahrenheit. Happiness drops as temperatures either rise or fall beyond 57 degrees. Sunshine was actually negatively associated with happiness, but it became unrelated when controlling for the time of day (because people are happier at night). However, sunshine had a positive effect on measures of sadness and depression. Humidity was slightly negatively correlated with happiness, while wind speed and precipitation had no relationship.
57 degrees seems pretty low to adjust your thermostat to, but you might want to consider turning it down knob at the least somewhat. This will not only save you money in the winter, but according to this study it will make you happier as well.
Also, this is good news heading into the fall!
Tsutsui, Yoshiro. “Weather and Individual Happiness.” Weather, Climate, and Society 5.1 (2013): 70-82.