When Yom Tov begins right after Shabbos (as with Shavuos of this year), can one light a 2-day Yizkor candle before Shabbos and have it continue burning until Yomtov, or there is no purpose in doing so because the candle was not lit on or just prior to the Yizkor day?
Would A candle lit for a Yohrtzeit have the same rules as a Yizkor candle?
If one may light it even 2 days before, is this Lechatchila anytime, or only when it cannot be done otherwise, as in the case of Shabbos?
Rav Schachter writes:
Some poskim question whether one is permitted to light a yahrtzeit candle on the second day of Yom Tov in honor of Yizkor. The Kitzur Shulchan Aruch (98:1) cites poskim on both sides of the issue and concludes that one should not light such a candle on Yom Tov. He views it as a “ner shel avtala,” a candle whose light does not provide benefit on Yom Tov, and as such may not be lit. Therefore, one should ideally light a yahrtzeit candle on erev Yom Tov.
The book “Mourning in Halacha” does not mention your specific case.
But it does say that if the Yahrzeit falls on Yom Tov and one forget to kindle the light on the preceding afternoon, one should kindle it from an existing flame in the manner permitted on Yom Tov in the room where people eat, so that they will benefit from the light. But it is more correct to kindle it in the synagogue; however under pressing circumstances, it is possible to permit kindling it elsewhere. (He quotes the Biur Halocho 514 s.v. Ner shel batalah).
I see from here that one can light the Yahrzeit light on Yom Tov itself which offers you a practical solution, if not an answer to your direct question.
On the purpose of the Yahrzeit light, the end of the Biur Halocho reads,
It may be possible to allow the Yahrzeit light anyway because it is a bit like a light for a mitzvah because it is for the honour of his ancestors – so it is explained by the Ksav Sofer.
