There is evidence to support that during the Spring and Summer months, people are simply happier than they are during the Fall and Winter months.
The idea behind this theory is that the production of serotonin and melatonin changes throughout the year based on the hours of sunlight that you experience in a single day.
In the Winter, days appear to be shorter because the skies go darker earlier, while in the Summer, days appear to be longer because the skies go darker later.
What you get as a result of this is an increased production of melatonin in the Winter, which makes you feel sleepier, and even sadder. In the Summer, however, you see an increased production of serotonin, which in turn, makes you feel happier and more alert.
There have also been studies to suggest that seasonal moon changes don't affect everyone, but does affect a significant part of the population.