Open For Business
noun
営業 means running a business. You're in the middle of running a business, which means you're open for business (otherwise you wouldn't be in the middle of running it, you'd be at the beginning or something).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will help to know 営業 first.
今日は日曜日なのに営業中なんですね。
It's open for business today, isn’t it, even though it's Sunday.
営業中に電話しないでって言ったじゃない。
I asked you not to call me when the shop is open.
営業中にお客さんの前でヒジをつくなんて、一流のホストだったらありえないよ。
It's unthinkable for a top-class host to lean on their elbow in front of customers during business hours.