After Tachanun, (on Mondays and Thursdays following the Torah reading), three passages of prayer are recited.
The first is Ashrei (Tehillah L’David). Although this prayer has already been recited in Pesukei d’Zimrah, the Psalm is repeated, for the Chachamim say (Berachot 4b), “Whoever recites Tehillah L’David three times daily is promised life in the World to Come.” First it is recited in Pesukei d’Zimrah; a second time after Tachanun, and a third time before Minchah (see the laws of Pesukei d’Zimrah earlier in this book 14:3 and note 5).
Afterwards, we recite the Psalm “Lamenatze’ach mizmor l’David, ya’ancha Hashem b’yom tzarah” (“For the One Who grants victory, a Psalm of David. May Hashem answer you on the day of distress”), which serves as a continuation of the prayers of supplication (Tachanunim) recited after the Amidah.
Since Lamenatze’ach is a prayer about a day of distress, it is not recited on days of joy. There are various customs surrounding this Psalm. According to the minhag of the Sephardim, the law concerning it is like that of Tachanun, and every day on which Tachanun is not recited due to the joy of that day, Lamenatze’ach is not recited either. According to the Ashkenazic minhag, in order for Tachanun to be omitted, a minor joyous occasion is sufficient; however, for Lamenatze’ach to be omitted, the joyous occasion must be great. Therefore, when a chatan or a ba’al brit (the father, the mohel, or the sandak) is praying with the congregation, Tachanun is not recited, yet Lamenatze’ach is. Likewise, regarding the month of Nisan, and the days from Yom Kippur until the end of Tishrei, although Tachanun is not recited, Lamenatze’ach is. The only times it is not recited are on holidays, the eve of holidays (erev chag), and the day following a holiday (isru chag) (Rama 131:1; Mishnah Berurah 35; Kaf HaChaim 37). These customs are printed in the siddurim before the Lamenazte’ach paragraph, and each ethnic group follows its individual custom.