Rabbi Hershel Schachter
Many misunderstand the minhag (custom) of reciting yizkor as representing a solemn few moments of sadness. There is a universal minhag that one who has both parents alive leaves before yizkor. This too is misunderstood as representing the idea of al tiftach peh lasatan. Since those reciting yizkor are participating in an act of aveilus (mourning), we don't want those whose parents are alive even to be present, so as, cholila, to imply as if they too are in mourning. Whenever yizkor is recited the practice in many shuls is that an appeal is made for some charity or another. This minhag too is usually misunderstood. Many assume that since many more people show up for yizkor than on other days of the year, we have a captive audience which presents a better opportunity for an appeal.
All three assumptions are incorrect! Yizkor is always recited on Yom Tov, when there is a mitzvah of simcha. Aveilus and simcha are mutually exclusive. One may not observe any forms of mourning on yom tov.
In the times of the Baalei haTosafos, when the tfillah of yizkor was instituted, the same number of people would be present in shul on the weekdays as on Shabbos and yom tov.
The Yizkor appeal was not instituted "after the fact", because so many people were reciting the yizkor prayer, but rather as an expression of simchas yom tov, an appeal for the poor was introduced on yomim tovim. Rambam writes (Hilchos Yom Tov 6:18) that one who eats and drinks on yom tov and doesn't share with the poor is merely engaging in "simchas kreiso - the rejoicing of his stomach". The Torah defines simcha as one who is mesameach others who are less fortunate, such as orphans, widows, and converts. The yom tov appeal was always for the poor and needy. Once people were pledging for tzedaka, as a method of fulfilling simchas yom tov, the yizkor prayer was introduced: let this pledge be considered as a zechus (merit) for one's parent(s) who raised a child with proper attitudes and values regarding sharing of their assets with others. And the reason those who didn't recite yizkor leave the shul is because the Talmud mentions that it doesn't look right when everyone in shul is praying and one individual abstains. The mistaken impression conveyed is that perhaps that individual doesn't belive in the power of tfillah.
We just celebrated Purim. Two of the special mitzvos of that holiday are mishloach manos and matanos laevyonim. Rambam writes (Hilchos Megillah 2:17) that if one can afford to go above and beyond the basic obligation of these two mitzvos, it would be preferable to give extra matanos laevyonim as opposed to placing the extra emphasis on the mishloach manos.
"There is no more glorified form of simcha than to cheer up the hearts of the orphans, widows, and converts; the one who cheers up the hearts of these unfortunate individuals is to be compared to G-d Himself"
In recent years some have started a new and most meaningful and beautiful minhag: when spending lots of money here in America on our bar mitzvas or weddings, in order to enhance the simcha, they will sponsor a bar mitzvah or wedding on behalf of those who can not afford to make one on their own (or, alternatively, contribute in another way to tzedaka). This is the most glorious method of engaging in simcha.
This fact even affected observance of halacha. See Tosaofs Gittin(59b) s.v. aval.
Brachos (20b). See Nefesh Horav p.153