The times for lighting a Yahrtzeit candle, coinciding with Yizkor at services, are as follows:
Erev Yom Kippur
Erev the Seventh Day of Pesach*
Erev Shemini Atzeret.*
*Those who observe a Second Festival Day (Or Shalom does not) light the following night.
(when any of these occurs on Saturday night, we must wait until after nightfall before lighting)