Whether consciously or unconsciously the mind remembers the anniversary dates of painful events in our life. The really emotionally upsetting events will be seared into our minds and the dates are obvious. But it is not just that you feel sad, mournful on those dates. Almost a week leading up to that date, your moods may change. Sometimes the mind regards the whole month as a morose month.
At times we are not aware of the anniversary of the event consciously but subtle changes in the mood would have started. We may not be able to figure out the cause until we look deeper. Death anniversaries of loved ones are obvious. It does not matter how many years have passed. Our minds still become melancholic on those dates and the days leading to those dates. People ask for how many years will we feel this? Perhaps it depends on how deep the love and bond was between the two people. It may also depend on the outlook of a person towards death, dying, belief in afterlife etc.
But such melancholic replay of moods is not restricted to death anniversary alone. It can be any painful event in our lives. People who have gone through a messy divorce will feel it when the anniversary of the divorce date approaches. Some are so aware of the date, they dread as the date approaches every year.
The anniversary of the day someone lost valuable goods, property, or experienced great shame or humiliation can also trigger mood changes. The dates of forceful relocations can also be another triggering factor. The dates when the trust was betrayed also have a place in the mind.
Because most of us follow the Greco-Roman calendar of January-December, our dates are remembered on this calendar. When we are aware of the anniversary dates of adverse events of our loved ones, we can take extra care around them. Knowing that they will be melancholic, low-energy etc helps us to be prepared for it. If we are supportive for that week, the phase will pass and life gets back to normal.
If you are feeling unusually sad with no immediate reason perhaps, looking back might help to make sense of that in some cases.