According to the view of the Shulhan Aruch, it is permissible to recite the "Siduk Hadin" service in memory of a deceased person on Rosh Hodesh, Hanukah, Purim, Hol Ha’mo’ed, Ereb Shabbat and Ereb Yom Tob, and throughout the month of Nissan. Although these are festive occasions when the solemn Tahanun prayer is omitted, the Shulhan Aruch nevertheless maintains that one may recite the "Siduk Hadin" on these days. Common practice, however, is to refrain from conducting the "Siduk Hadin" service or making eulogies on these days. (On Fridays, the practice is to refrain from reciting the "Siduk Hadin" after midday.)
The one exception to this rule is eulogies delivered at the funeral of a Torah scholar. Eulogies in such a case may be delivered on the occasions listed above. Although the Magen Abraham (Rabbi Abraham Gombiner, Poland, 1637-1683) writes in Siman 547 (Se’if Katan 12) that scholars nowadays do not have the formal Halachic status of a "Talmid Hacham," common practice follows the view of other authorities, that this status applies even to today’s Torah scholars. Therefore, eulogies may be delivered at the funeral of a Torah scholar on Ereb Shabbat, Ereb Yom Tob, Hol Ha’mo’ed, Hanukah, Purim and Rosh Hodesh, and during the month of Nissan.
It must be emphasized that this applies only at the funeral, in the presence of the deceased. Eulogies may not be delivered on these occasions if the body of the deceased Torah scholar is not present.
Rav Shemuel Pinhasi, in his work Haim Va’hesed (4:8), addresses the status of assemblies held in memory of a Sadik after his passing. Occasionally, gatherings are held after a Torah sage’s passing for the purpose of inspiring people by speaking of the great qualities of the deceased rabbi. Would such a gathering be permissible on Rosh Hodesh, or on the other occasions listed above? (This question recently arose when an assembly was held on Rosh Hodesh Tammuz in memory of the former Chief Rabbi of Israel, Rav Mordechai Eliyahu zs"l.) Rav Pinhasi rules that these kinds of gatherings are allowed on days when Tahanun is recited, and even on Shabbat and Yom Tob. He explains that these assemblies, unlike eulogies, are not intended to evoke feelings of anguish and sorrow. Their purpose is to inspire people, to speak about the deceased Sadik in order to perpetuate his legacy. This is not a sad event by any means, and is therefore wholly appropriate for festive occasions such as Shabbat and holidays. These gatherings differ from eulogies, which are intended to evoke feelings of grief and cause people to cry over the loss of the deceased. Eulogies are obviously incongruous with the festive nature of Shabbat and holidays, and are therefore forbidden on these occasions (with the exception of funerals for Torah scholars, during which, as discussed, eulogies are permissible on the days listed earlier). Memorial gatherings, by contrast, are not sorrowful events, and may therefore be held even on Shabbat and Yom Tob.
Summary: It is customary not to deliver eulogies or recite the "Siduk Hadin" service on Rosh Hodesh, Hanukah, Purim, Hol Ha’mo’ed, Ereb Yom Tob, after midday on Ereb Shabbat, or throughout the month of Elul. At a Torah scholar’s funeral, however, eulogies may be delivered on these occasions and the "Siduk Hadin" may be recited, if the deceased is present. It is permissible to conduct gatherings in memory of a deceased Torah sage for the purpose of inspiring people to continue his legacy, even on Shabbat and Yom Tob, and certainly on the occasions listed above.
