In the diaspora, we say Yizkor on:
- The last day of Passover
- The second day of Shavuot
- Yom Kippur
- The first day of Shemini Atzeret
Why do we always say Yizkor on these days in particular? If it's because they are the last days (understanding Rosh Hashanah and Yom Kippur as one holiday, and Sukkot-Shemini Atzeret as another), why not Simchat Torah?
My cynical assumption is that it is a combination of length of service and to fill seats in shul, but I have no sources.