Yizkor, a prayer in which we remember parents who have died, is said before the mussaf service in shul on Yom Kippur, Shemini Atzeret, the last day of Pesach and Shavuot.
It is not very widely known that the core of Yizkor is committing to give charity in your relative’s memory (after the holiday). The purposes, then, of Yizkor are to remember loved ones and add to their merits in the world to come by doing good deeds as their descendants in this world.
Many people customarily leave shul for Yizkor if they are blessed to have both parents still living. This is not necessary and is based on a folk belief that staying in may attract the ayin hara, the evil eye.
When I was small, the numbers swelled at Yizkor time and then dwindled straight after. There were “Yizkor Jews” who turned up to remember their parents but were almost never seen in shul otherwise. My impression is there are fewer of them today. It is hard to sustain a Judaism based on reverence for one’s ancestors if it is not meaningful and alive for you.