תְּקַעוּ בְּיַרְחָא דְתִשְׁרֵי שׁוֹפְרָא בְּיַרְחָא דְמִתְכַּסֵי יוֹמֵי חַגַיָא דִילָנָא.
1. on the moon's covering | בַּכֵּסֶה – The word appears only here and in Mishlei 7:20 (where it is spelled "כֵּסֶא"). See Bavli Rosh HaShanah 34a that it relates to the root "כסה" (to cover), and refers to the day that the moon is covered, i.e. the new moon. The word, thus, is parallel to "בַחֹדֶשׁ" of the first clause. Rashi and others alternatively maintain that the word means: "the set day", pointing to this usage in Mishlei. Hoil Moshe reaches the same conclusion but suggests that the word might relate to the noun "מִכְסָה" (Shemot 12:4), a computation. Cf. Shadal that the verse refers to the day of the Pesach sacrifice, in the middle rather than the beginning of the month.
2. for our festival day | לְיוֹם חַגֵּנוּ – Bavli Rosh HaShanah 34a learns from this phrase that the verse is speaking of Rosh HaShanah, for this is the only festival that falls out on the new moon. However, the verse appears more simply to be referring to the festival of the new moon itself.