Psalms 81:4

תִּקְעוּ בַחֹדֶשׁ שׁוֹפָר בַּכֵּסֶה לְיוֹם חַגֵּנוּ

Blow shofar at the new moon, at the appointed time for our day of celebration.

The word keseh (KSH) is what’s called a hapax legomenon, a Greek phrase for a word that only occurs once in a text. Since this word only occurs once in all of Tanach, its meaning is obscure. The Metzudos explains that it means a time that is designated for a particular holiday, namely Rosh Hashana. He compares the word keseh to hakeiseh (KSA) in Proverbs 7:20, the letters alef and hei being interchangeable. (The former comment appears in Metzudas David, the latter in Metzudas Tziyon.)