I know that it’s customary to lead prayers the Shabbat before a Yahrzeit. Does this apply when the Yahrzeit is on a Yom Tov when the mourner will also be leading prayers?
Shalom, Thank you for your question. The reason why there is a custom to lead the service on the Shabbat before the Yahrzeit is to add merit to the departed soul before the day of the Yahrzeit when the soul is (re)judged. Based on this there are those who believe that it is more important to lead the service (or read the Haftorah) on the Shabbat before, than even leading the service on the Yahrzeit itself. Others though disagree, and believe that if one can only lead the service or be called up on one of the days, then it is best to choose the day of the Yahrzeit itself. So, in answer to your question – yes, even if the mourner will be leading the service on the day of the Yahrzeit and be getting a call up to the Torah then, on Yom Tov in your case, it is still correct to lead the service on the Shabbat before. However, in many synagogues there are restrictions on who may lead the service and how often. These local rulings are practiced in order to make sure that the services are well run, and that the honors of leading the service are distributed fairly. Because of this there may be limitations on allowing someone to lead the service on the Shabbat before the Yahrzeit and on the Yahrzeit itself. In such a case, one should probably choose to lead the service on the Yahrzeit – although one should always ask the local Rabbi and follow his ruling. Blessings.