It is customary to light a yahrtzeit candle for the souls of departed relatives when Yizkor is said on Yom Tov. This year, it should be lit on Friday, rather than on Yom Tov eve, since lighting such a candle is considered `ner shel batala,' whose light does not enhance the festival and is therefore forbidden to be lit on Yom Tov itself (see Biur Halacha, Siman 514, Se'if 10).
This year Shavuos falls on Sunday and Erev Yom Tov is Shabbos. Therefore, it is necessary to light a yahrtzeit candle before Shabbos and see that it can burn until after Yom Tov, on Sunday night, that is, two days later.
A forty-eight hour candle must be bought. If one has not prepared such a candle, he can take an empty tin can, place a regular 24-hour yahrtzeit candle in it [just the candle itself] and add enough oil to last the 48 hours before lighting it on Friday.
Bedi'eved, if he only remembered this on Yom Tov itself, he may light a conventional yahrtzeit candle, taking fire from an existing source, but only in the memory of parents, not other relatives, and place it where its light can provide benefit on Yom Tov.
This applies to residents in Israel, where Yizkor is said on Sunday. Those living elsewhere should ask a competent halachic authority for guidance.
