So what would they commemorate and why? Are there official ceremonies (such as Remembrance Sunday, Veterans' Day etc)? Are there things that are deliberately ignored - history that individual characters or indeed your whole created world is determined to forget (almost inevitably because it won't show them up in a good light)?
What rituals are used by individual characters/your fictional world? What happens to those who prefer their own rituals? Do they have the freedom to do that or are they punished for it as people are expected to fit in? On a sadder note, what rituals are used for saying goodbye to loved ones and remembering them after death? On a happer occasion, how are birthdays, weddings, births etc celebrated? What do your individual characters make of these things (their reactions can reveal so much about them)?