Yizkor by all sources is an ancient custom. The earliest source for Yizkor is found in the Midrash Tanchuma, Haazinu in which it cites the custom on Yom Kippur of recalling loved ones who have since departed and pledging charity on their behalf. The Ashekenazic custom of reciting Yizkor on the 3 pilgrimage holidays when all Jews would go to Jerusalem include Sukkot, Shavuot, and Pesach, was probably of a later origin, possibly during the times of the Crusades. These three festivals are also times that the Torah ordained to give charity since no man could appear at the Temple on these festivles empty-handed. This is probably why yizkor is present on these days since in yizkor we remember the lives of the departed and we pledge charity on their behalf. Our praying and our charity will bring them an elevation in the next world.