The final words of the Maftir on Rosh Hashana describe the offerings in the Beis Hamikdash as a reiach nichoach, a fragrance pleasing to Hashem. Rashi (Vayikra 1:9) explains the phrase "reiach nichoach" as follows: "Nachas ruach l'fanai, she'amarti v'naaseh retzoni - contentment before Me, that I said, and My will was done."
What is the literal meaning of nachas ruach? Ruach means spirit, typically that of people (e.g. Bamidbar 11:17, "the spirit upon Moshe"), but also, as it were, that of Hashem (e.g. Bereishis 1:2). Nachas means resting of, the construct form of nach, as in (Shemos 20:11) "He rested (vayanach) on the seventh day." When we wish people nachas, it usually means contentment related to children. Nachas is shorthand for nachas ruach, which is felt by parents who are blessed with children who fulfill their will. The mitzvah of kibud av v'eim, honoring one's parents, includes giving them nachas ruach (see Responsa of Rav Akiva Eiger I:68). A child must, to the extent possible, do anything reasonable (see Chazon Ish Yoreh Deah 149:8) that his parents wish, even if they have not commanded him to do so. The Ramban (Shemos 20:12) comments that the Torah does not specify the details of how one honors parents, and this is derived from the aforementioned honor due to Hashem, our Father. We must try to fulfill the will of Hashem, even when there is no explicit command.
The Ramban (Sefer Hamitzvos, shoresh 1) limits the Torah's requirement to fulfill the rulings of Chazal (Devarim 17:11) to rabbinic interpretation, but not to rabbinic innovation. Rav Elchonon Wasserman (Kuntres Divrei Sofrim 1:15) asks: why, then, are we obligated to observe rabbinic commandments? He answers (1:17,18) that Chazal, like Moshe Rabbeinu before them, were able to intuit the will of Hashem (see Yevamos 62a). It is Hashem's will that we fulfill rabbinic law. For this reason, we are obligated to do so.
On the second day of Rosh Hashana, when we fulfill the rabbinic command to blow the shofar on yom tov sheini, we read the story of akeidas Yitzchak. Hashem tells Avraham "please take" Yitzchak and bring him up as an offering (Bereishis 22:1). The expression "please", according to the Drashos haRan (6), implies a mere request, not a commandment. Had Avraham refused, he would not have been punished, as he was not commanded to do the akeida, but merely requested to do it. Yet he passed the test by performing the akeida in order to fulfill the will of Hashem. Indeed, when we invoke the merit of akeidas Yitzchak on Rosh Hashana at the end of Zichronos, we say that Avraham did Your will wholeheartedly (la'asos retzoncha b'levav shalem). Our founding father gave the ultimate nachas to our Father in Heaven by fulfilling "I said, and My will was done."
One can obey commandments even halfheartedly, as is common in both human and Divine commands. When one goes beyond the call of duty and does what is known to be the will of Hashem, a parent, or another, it is typically done wholeheartedly.
In the opening paragraph of the addition to Shemone Esrei on Rosh Hashana and Yom Kippur, (u'vchein tein pachdecha), we pray for the time when all of mankind will unite "to do Your will wholeheartedly". Each one of us can hasten this ultimate era of redemption by doing so ourselves. May we all give nachas to our Father in Heaven by doing His will, and, thereby, be blessed with a ksiva vachasima tova.
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