The Laws of Candle-Lighting on Rosh Hashanah
On the first day of Rosh Hashanah, we customarily light Yom Tov candles before the onset of Yom Tov similar to the way we light them on Erev Shabbat. If the candles were not lit before the onset of Yom Tov, a woman may even light the candles on Yom Tov itself, provided that this is done in a permissible fashion, i.e. by transferring the fire from an existing flame and lighting the candles; it is prohibited to ignite a new flame on Yom Tov, as we have previously explained in the laws of Shavuot.
The Proper Time to Light Candles for the Second Day of Yom Tov
On the second day of Rosh Hashanah as well as any other second day of Yom Tov in the Diaspora (meaning the second day of Yom Tov celebrated outside of Israel for every holiday, just as two days of Rosh Hashanah are celebrated even in Israel), Yom Tov candles are lit as well.
Some have the custom to light the candles for the second day of Yom Tov of Rosh Hashanah after nightfall (i.e. approximately twenty minutes after sunset) before reciting Kiddush on a cup of wine.
The reason for the above custom is because it is forbidden to prepare anything on the first day of Yom Tov for the second just as it is forbidden to prepare anything on Shabbat or Yom Tov for weekday use. Based on this, it would seem that it is forbidden to light candles “in honor of the second day of Yom Tov” when it is still during the first day of Yom Tov.
Indeed, it is correct that one may not prepare anything on the first day of Yom Tov for the second and it is therefore prohibited to prepare the wicks, cook, or wash dishes on the first day of Yom Tov for the second for this is considered “preparing.”
Nevertheless, regarding candle-lighting, Maran Rabbeinu Ovadia Yosef zt”l writes (Chazon Ovadia-Yamim Nora’im, page 182) that based on the words of the Tosafot and other great Poskim, one need not be concerned about the prohibition of preparing and one may light candles for the second day of Yom Tov even before sunset of the first day of Yom Tov, for this serves a purpose for the first day of Yom Tov as well since the candles add light to the house. He quotes Rabbeinu Yosef Haim (in his Responsa Rav Pe’alim, Orach Chaim, Volume 4, Chapter 23) as ruling likewise.
Summary: Candles are lit in honor of Rosh Hashanah just as they are lit in honor of every Yom Tov: Sukkot, Pesach, and Shavuot. The candles should preferably be lit before sunset. It is likewise permissible to light candles for the second day of Yom Tov before sunset by using a pre-existing flame.
This year, Rosh Hashanah of the year 5775 will fall out on Thursday and Friday. As such, we must explain the proper of how to light candles on the second day of Yom Tov in honor of Shabbat in the following Halacha.