A Case Study of Custom and Meaning in the Siddur: Mizmor L’Todah

(1) A psalm for praise.^-a

Raise a shout for the LORD, all the earth; (2) worship the LORD in gladness;
come into His presence with shouts of joy. (3) Acknowledge that the LORD is God;
He made us and we are His,^-b
His people, the flock He tends. (4) Enter His gates with praise,
His courts with acclamation.
Praise Him!
Bless His name! (5) For the LORD is good;
His steadfast love is eternal;
His faithfulness is for all generations.

Rabbi Pinchas and Rabbi Levi and Rabbi Yochanan said in the name of Rabbi Menachem of Gallia: In the Time to Come all sacrifices will be annulled, but that of thanksgiving will not be annulled, and all prayers will be annulled, but that of Thanksgiving will not be annulled. This is indicated by what is written, “The voice of joy and the voice of gladness, the voice of the bridegroom and the voice of the bride, the voice of them that say: Give thanks to the Lord of Hosts” (Jeremiah 33:11)--this refers to Thanksgiving; “That bring offerings of thanksgiving into the house of the Lord” refers to the sacrifice of thanksgiving. So too did David say: “Your vows are upon me, O God; I will render thanksgivings unto You” (Psalms 56:13). It is written here not “a thanksgiving” but “thanksgivings,” meaning Thanksgiving in prayer and the sacrifice of Thanksgiving.

(9) "A Psalm of Thanksgiving" (Psalms 100) should be said in a melody because in the future all of the songs will be cancelled except for a Psalm of Thanksgiving. Rem"a: And we do not say "A Psalm of Thanksgiving" on Shabbat or holidays [Tur] or on the [intermediary] days of Pesach, because the Thanksgiving offering was not sacrificed on those days on account of Chametz. And it should not be recited on the eve of Passover... And not the eve of Yom Kippur... And thus it has become customary in our lands.

“On Account of Chametz”- because with the Thanksgiving offering they needed to bring ten leavened loaves of bread. Even on Passover Eve they were not able to bring the Thanksgiving offering lest they not be able to eat it by the time the prohibition on eating chametz begins, and would need to burn it, and it is forbidden to cause sanctified things to become burnt, and for this reason also on Yom Kippur evening, due to the short time available for eating, [and leaving them uneaten] would make the sacrifices unsuitable.

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The Rem’’a says “And we do not say "A Psalm of Thanksgiving" on Shabbat or holidays or on the [intermediary] days of Pesach, because the Thanksgiving offering was not sacrificed on those days on account of Chametz, etc…” but this not the main reason [we say the Psalm in the first place]. [in another place, it says] “we do not say this Psalm in place of the Thanksgiving sacrifice, rather for the sake of giving thanks…” our custom is to say it on Passover eve and on the intermediary days of Pesach.