The Peak-end Rule dictates that people tend to judge an experience based on how they felt at its peak and at its end, rather than the sum or average of every moment of the experience. This is why a particularly bad flight home from an other wise fantastic trip can leave you with a negative impression of the whole thing.

This could also be a positive and productive day of work that ended with a frustrating meeting, leading to a bad feeling about the whole day bleeding into the evening or weekend.

It's a crude patch over a deeper problem, but a quick review of a day or week of work to summarise the good achievements can help to avoid dwelling on the bad. It's also why horrible musicals often close with a horrible medley of all the rousing songs the viewer had been subjected to over the previous 90 minutes or so.