There is a widely accepted practice that we refrain from reciting Kel Malay Rachamim during the month of Nissan. In many Shuls, the Shabbat preceding Rosh Chodesh Nissan includes a long line of those asking to make a Kel Malay Rachamim as they may be observing a Yahrtzeit sometime during the month. When faced with a Kevura on Chol Hamoed we refrain from eulogizing. Yom Tov is Mevatel Aveilut and the prohibition to eulogize even spills over to the rest of the month of Nissan. And yet, we recite Yizkor on the last day of Pesach. Doesn’t its recitation seem somewhat incongruous with the spirit of the day?
The Rav, zt”l, often highlighted two different types of Mitzvot. There are some Mitzvot in which the Kiyum Hamitzva and the Maase Hamitzva are identical. Both the Maase Hamitzva and Kiyum Hamitzva of Matza is to eat a Kizayit of Matza. The same is true of Tefillin and Lulav. However, there are some Mitzvot in which the Maase Hamitzva is not equated with the Kiyum Hamitzva. Regarding Shofar the Maase Hamitzva is the Tekiya but its Kiyum is one of Tefilla. Fasting on a Taanit Tzibur is a Kiyum of the Mitzva of Teshuva. When one observes Purim or Pesach, the Kiyum is the Mitzva of Vinikdashti Bitoch Bnai Yisrael. So too vis-à-versa the Mitzva of Simchat Yom Tov. The Maase Hamitzva is manifested by consuming meat and wine but the Kiyum is an internal one: Lihiyot Sameyach Vitov Lev.
It is difficult for me to say this because I, Baruch Hashem, have not found it necessary to recite Yizkor as yet. But I would humbly suggest that the inner feelings associated with Yizkor should be one of Simcha. There are those who define the Mitzva of Peru Urevu as not only having children but grandchildren. After the grandchild is born the grandparent fulfills his Mitzva of Peru Urevu. In relationship to Talmud Torah, our obligation is Vehodatem Livanecha Ulivnei Vanecha Yom Asher Omedet Bachorev. Similarly, in the context of Sipur Yetziat Mitzrayim, the Torah states Ulimaan Tisaper Biaznei Bincha Uvein Bincha Et Asher Hitalalti Bimitzrayim. When parents and grandparents see their children and grandchildren participate at a Seder and fulfill the Mitzva of Shalosh Regalim Tachog Li Bashana, they can rest secure in the knowledge that they have succeeded in their mission of Vihodatem and Ulimaan Tisapru. This knowledge will assure the fact that Bigan Eden Tihay Menuchatam. Although the Minhag to recite Yizkor is one of sorrow, the Kiyum of the Mitzva is one of inner Simcha and joy, and hence is appropriately recited on Pesach.
Additionally, our collective Kel Malay on behalf of the Kedoshim who perished during the Holocaust or the Kel Malay recited on behalf of members of Chayalei Tzahal who passed away Al Kiddush Hashem could be viewed in this light. As they dwell in the Yeshiva Shel Mala and see that not only has Judaism survived the attempts of Hitler, Yimach Shimo, but has flourished and prospered and seen a revitalization of Torah study, this brings a sense of inner joy to their Neshamot and should bring Simcha and Nechama to those reciting Yizkor.