help! 約定的日子就到,可否用 our gathering is approaching tomorrow

Gathering is a bit strange, English people won't say like that. If you
really want to use that word, then it should be
"Our gathering is on tomorrow."

I think it should be better to say:
"We will meet tomorrow."
"We are going to meet tomorrow."

You do not have to translate the sentence word by word, you can simply say, Our gathering is tomorrow. "Approaching" is unnecessary.

用 coming 會好d
approach 比較aggressive, 解作某人或某物的接近

our gathering is coming