en_tn/lev/23/07.md

The first day you must set apart to gather together

"You must set apart the first day to gather together" or "You must treat the first day as different and gather together"

will present a food offering

They would present it to Yahweh by burning it on the altar.

The seventh day is an assembly set apart to Yahweh

The requirement that people assemble on that day is spoken of as if that day were the assembly. Being set apart to Yahweh means that when they assemble, they must worship Yahweh. AT: "The seventh day is a day when you must assemble together to worship Yahweh" (See: rc://en/ta/man/translate/figs-metonymy)