After the holidays, a drop in mood may occur for several reasons, linked both to the contrast between the emotionally rich moments of celebration and everyday life, as well as to individual differences in personal perception.

Post-Holiday Emotional Paradox

After the holidays, a drop in mood may occur for several reasons, linked both to the contrast between the emotionally rich moments of celebration and everyday life, as well as to individual differences in personal perception. During the holidays, people experience intense positive emotions that sharply elevate their mood; however, returning to the regular pace of life is often perceived as a loss of that magic and leisure. This disparity in emotional intensity can lead to feelings of emptiness, melancholy, or even mild depression.

In addition, the individual’s inner disposition plays an important role. As noted in one source, external triggers (such as holidays or tragedy) can only provoke sadness, but the true cause always lies within the person—in their perception and emotional resilience. This is confirmed by the following statement:
"Although sadness can be triggered by grief or sorrow, heavy events are not the cause of sadness; they merely provoke it. The true cause is always within the person—how they perceive the events of life." (source: link txt)

There is also a perspective that emphasizes the need for a balance between joy and a slight touch of sadness. During celebrations, people feel joy, and a small measure of sadness can, on the contrary, balance emotions, preventing those celebrating from drowning in an illusory bliss. However, when this element of balance is lost—that is, when even a minimal emotional hint of sorrow is absent after the holidays—a sense of emptiness may arise:
"True celebrations are needed for a grieving person, and a drop of sadness is necessary for the celebrant. Sadness without celebrations is a path to suicide, and celebrations without a trace of sadness are sheer madness." (source: link txt)

Thus, the post-holiday drop in mood is due to a combination of the stark emotional contrast between the festive period and ordinary life, as well as the individual’s internal perception and method of processing emotional stimuli. Together, these factors can lead to a period of reduced activity and a loss of the previous intensity of emotion following a phase of extreme emotional saturation.

Supporting citation(s):
"Although sadness can be triggered by grief or sorrow, heavy events are not the cause of sadness; they merely provoke it. The true cause is always within the person—how they perceive the events of life." (source: link txt)
"True celebrations are needed for a grieving person, and a drop of sadness is necessary for the celebrant. Sadness without celebrations is a path to suicide, and celebrations without a trace of sadness are sheer madness." (source: link txt)