In Israel where there are seven days of Chag on Sukkot and Pesach, Yizkor is said on the seventh day. Outside of Israel where eight days are celebrated, Yizkor is said in the eighth day.

When does one say Yizkor if they are from outside of Israel visiting Israel over the chag or vice versa?

Firstly, it is important to note that it is permissible to say Yizkor alone in a place where one has no Synagogue or minyan.

One should say Yizkor in accordance to where he lives. If visiting Israel than Yizkor is said on the eighth day, and an Israeli citizen visiting outside of the Israel should say Yizkor on the seventh day.