לֹֽא־תִטַּ֥ע לְךָ֛ אֲשֵׁרָ֖ה כׇּל־עֵ֑ץ אֵ֗צֶל מִזְבַּ֛ח יְהֹוָ֥ה אֱלֹהֶ֖יךָ אֲשֶׁ֥ר תַּעֲשֶׂה־לָּֽךְ׃
You shall not set up a sacred post — any kind of pole beside the altar of the Eternal your God that you may make.
Torah Temimah points to Yerushalmi Megillah 1:4.
Sefaria · Yerushalmi Megillah 1:4
תניא, יום הקהל שחל להיות בשבת מאחרין עד לאחר השבת, מאי טעמא, מפני הבימה, ולמה לא יעשוה מאתמול, א״ר מתנה, על שם לא תטע לך אשרה כל עץ.
English: It was taught: If the day of Hakhel falls on Shabbos, they postpone it until after Shabbos. What is the reason? Because of the platform. And why may they not build it the day before? Rabbi Matnah said: Because of “You shall not plant for yourself an asherah — any tree.”
The prohibition of any tree near the altar is extended by asmakhta to forbid building a wooden platform in the Temple that would stand over a festival and Shabbos, creating an appearance of a permanent wooden structure.
Derash on the verse (Yerushalmi) — not a formal named middah of the 13. The verse is used as an asmakhta for a practical restriction on Temple construction timing.
When Hakhel falls on Shabbos the ceremony is postponed so that the wooden platform need not stand over Shabbos, avoiding the appearance of a forbidden wooden structure near the altar.