Dear Rabbi Simon,
Am I allowed or obligated or encouraged to say Yizkor for my dad? What would be your advice?
Thank you for your Question. In view of the wording of the Yizkor prayer my feeling is that it is not appropriate for you to say Yizkor. However it would be a nice idea to give tzedakah in his memory before or immediately after yom tov. If you prefer to stay (rather than stepping out) for Yizkor you may do so. You can learn, read or recite tehillim during the few minutes while others are reciting the yizkor prayer.
Rabbi Rashi Simon