וְיָדַ֣עְתִּי אֱלֹהַ֔י כִּ֤י אַתָּה֙ בֹּחֵ֣ן לֵבָ֔ב וּמֵישָׁרִ֖ים תִּרְצֶ֑ה אֲנִ֗י בְּיֹ֤שֶׁר לְבָבִי֙ הִתְנַדַּ֣בְתִּי כָל־אֵ֔לֶּה וְעַתָּ֗ה עַמְּךָ֙ הַנִּמְצְאוּ־פֹ֔ה רָאִ֥יתִי בְשִׂמְחָ֖ה לְהִֽתְנַדֶּב־לָֽךְ׃
Ich weiß auch, mein Gott, dass du das Herz versuchst und Freude an der Aufrichtigkeit hast. Was mich betrifft, so habe ich in der Aufrichtigkeit meines Herzens all diese Dinge bereitwillig angeboten; und jetzt habe ich mit Freude gesehen, wie dein Volk, das hier anwesend ist, dir bereitwillig anbietet.
Sha'arei Teshuvah
Secondly - because on other holidays, we establish a meal for the joy of the commandment. For the reward for joy over the commandment increases and grows, as it is stated (I Chronicles 29:17), “now Your people, who are present here - I saw them joyously making freewill offerings.” And it is stated (Deuteronomy 28:47), “Because you would not serve the Lord your God in joy and gladness.” But since the fast is on Yom Kippur, they were obligated to fix the meal over the joy of the commandment, on the eve of Yom Kippur.