Musar su Salmi 118:15

It is stated, “When I celebrate the appointed Festival; I will judge [its theme] with fairness” (Tehillim 75:3). King Dovid was prompted to write these words because he recognized that our way is not that of the nations of the world, nor is our custom like theirs. For on their festive days they eat and drink to intoxication and go dancing in their public houses and commit other undesirable acts. But this is not the case with Israel, for although they also eat and drink and rejoice in the performance of the commandments, they are careful to spend half the day in prayer, expanding the service with special liturgies, after which they engage in study. All this is hinted at in the word “uprightly” [meisharim], as King Dovid said, “I will judge uprightly.” The mem [the numerical value of which is 40] of meisharim alludes to the forty days during which the Torah was given. And although Israel also rejoices on the Shabbos and Yom Tov, they do not neglect the three prayer services, morning, afternoon and evening. These are hinted at in the shi’en of meisharim [a shi’en is comprised of three vertical lines connected at the bottom]. And sometimes a shi’en is written with four heads [a four- headed shin appears on the side of the head tefillin]. This hints at the four prayers that are said on the festivals: morning, additional [mussaf], afternoon and evening. Moreover, we extend our prayers with additional poems and with the liturgical verses of the Krovetz [interspersed with the blessings of the Amidah on special occasions]. The word Krovetz itself is an acronym for the verse, Kol rinah viyshuah be’ohalei tzaddikim — “The voice of rejoicing and salvation in the tents of the righteous” (Tehillim 118:15). This is hinted at in the letters reish and yud of the word meisharim, which stand for rinah and yeshuah — “rejoicing and salvation.” The final mem of meisharim hints at the future redemption, which is alluded to in the phrase, lemarbei hamisrah — “for the increase of the realm” (Yeshayahu 9:6; the mem of lemarbei is uncharacteristically a final mem). This is to tell you that although we are filled with gladness and rejoicing on our festivals, nevertheless we pray for the promised redemption. For our happiness will only be complete when the Holy One Blessed is He rejoices with Tziyon and Yerushalayim.