en_tn/num/28/18.md

On the first day

This refers to the first day of the feast. This can be stated clearly. Alternate translation: "On day 1 of the feast" (See: rc://en/ta/man/jit/translate-ordinal and rc://en/ta/man/jit/figs-explicit)

there must be a holy assembly to honor Yahweh

"you must gather together to worship and honor Yahweh." The phrase "a holy assembly" means the people gather together to worship Yahweh. Worshiping Yahweh is a holy event.