וַיֹּ֣אמֶר נְחֶמְיָ֣ה ה֣וּא הַתִּרְשָׁ֡תָא וְעֶזְרָ֣א הַכֹּהֵ֣ן ׀ הַסֹּפֵ֡ר וְהַלְוִיִּם֩ הַמְּבִינִ֨ים אֶת־הָעָ֜ם לְכָל־הָעָ֗ם הַיּ֤וֹם קָדֹֽשׁ־הוּא֙ לַיהוָ֣ה אֱלֹהֵיכֶ֔ם אַל־תִּֽתְאַבְּל֖וּ וְאַל־תִּבְכּ֑וּ כִּ֤י בוֹכִים֙ כָּל־הָעָ֔ם כְּשָׁמְעָ֖ם אֶת־דִּבְרֵ֥י הַתּוֹרָֽה׃
And Nehemiah, who was the Tirshatha, and Ezra the priest the scribe, and the Levites that taught the people, said unto all the people: ‘This day is holy unto the LORD your God; mourn not, nor weep.’ For all the people wept, when they heard the words of the Law.
Rashi on Nehemiah
The day is holy because it is the day of Rosh Hashanah.
Rashi on Nehemiah
for all the people were weeping because they did not uphold the Torah appropriately.