Yerah ben yomo

Yerach ben yomo (יֵרֶח בֶּן יוֹמ֪וֹ, with variant English spellings, also known as Galgal), is a cantillation mark that appears only one time in the entire Torah (Numbers 35:5), and once in the Book of Esther. In these occurrences, it is followed immediately by a Karne parah, another mark that is found only once in the entire Torah. The symbol for this trope is an upside-down Etnachta.

The Hebrew words יֵרֶח בֶּן יוֹמ֪וֹ translate into English as day-old moon. Its alternate name גלגל translates into English as sphere.

The rare trope sound in the Torah appears in Numbers 35:5 on the word Alpayim (אלפים, two-thousand), which is followed by an equally exclusive Karne Parah on the word B'amah (באמה, cubit), in the first of four occurrences of this phrase in the verse. In each of the phrase's four appearances, a different set of trope. The Yerach ben yomo followed by the Karne Parah is found on the first of these four instances. On the other three, respectively, are a Kadma V'Azla, a Munach Rivi'i, and a Mercha Tipcha.

This is representative of the way mitzvot are performed in real life. When one first performs a mitzvah, being a new experience, it is performed with great enthusiasm. The unusual trope signify the one-time occurrence of the mitzvah being a new experience. The second instance is on a Kadma-V'Azla, a note that is recited highly, showing that the high is still alive. The third is on a Munach-Rivi'i, a note that is going downward, showing that enthusiasm is going down. The fourth and final occurrence being on a Mercha-Tipcha, a common set that are recited in a lazy mode as if they are basically being recited without a melody, show the monotony of performing a mitzvah after performing it so many times.

...
Wikipedia