One should remember to light a 24-hour candle so that one will have a fire with which to light the Yom Tov candles on the first and second nights of Yom Tov. In addition, those who recite Yizkor should light a 48-hour candle so that it is burning during the second day of Shavuot. If necessary, this can be lit on the second night of Yom Tov from a pre-existing flame. In this case, it is best to light it in a room where people will be eating.
The Lubavitcher Rebbe said that it is a good idea for young girls who are ready to start lighting Shabbat and Yom Tov candles to begin doing so on the Yom Tov of Shavuot (or the closest Yom Tov before their third birthday). By doing so, the bracha of shehechiyanu can also be referring to the mitzvah of the candles. If one wishes to start before the Yom Tov, it is best for the young girl to wear a new dress and say shehechiyanu while keeping in mind the mitzvah.
