"day 15 of month 7." The word "month" refer to those in the Hebrew calendar. (See: [[rc://en/ta/man/translate/translate-hebrewmonths]] and [[rc://en/ta/man/translate/translate-ordinal]])
"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.
This is an idiom. Here the word "keep" means to observe or celebrate. The word "him" refers to Yahweh. Alternate translation: "you must observe the festival for Yahweh" or "you must celebrate the festival for Yahweh" (See: [[rc://en/ta/man/translate/figs-idiom]])