A lot of people have difficulty with the "Sunday Blues," in part due to the association with going back to school, as zjw commented, and then later, back to work. It's definitely a thing.
It marks the end of another measured period of time without again accomplishing alll that we would have liked to accomplish, with many of the same old threads hanging loosely, and often with the nagging sense that we're still, and again, not quite where we're "supposed" to be.
Same thing happens after the holidays in January, with the time frame being much larger, and the regrets seeming more foreboding. Rates of depression and drinking, as well as attempted suicides,increase only AFTER the holidays for these and other reasons. The Holidays never live up to the hype, and often remind us of our loneliness.
The yearly ritual of making resolutions is an attempt to counter the unwanted feelings attached to having "fallen short" yet again the previous year. It's also a form of denial, given that most people are reluctant to admit such a thing about themselves, or just won't talk about it. Many people don't care about resolutions, so they don't get involved. Whereas some people don't make them out of a sense of futility.
It's a good thing to be aware of such things in order to better prepare ourselves for the letdown. I got into the habit this time around of getting up early on Sundays, cleaning and doing laundry, and then finishing last wee's work, and then preparing for next week's work at home. All by 1:00PM. I make time to read later on, but I do whatever else I want after one. Nothing wrong at all with doing something different in order to get yourself out of a rut.
Oh yeah...The desire to drink often strikes when we're most vulnerable, and Sunday Blues and Holiday Blues present the perfect storm.