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/translate/translate-ordinal]] and [[rc://en/ta/man/translate/figs-explicit]])
"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.