noun which may take the genitive case particle 'no'
open for business; now open
Typically used on signs at the entrance of a shop or restaurant to indicate it is currently open.
店の前に「営業中」の看板が出ていた。
There was a sign saying 'Open' in front of the shop.
営業中はいつでもお入りください。
Please feel free to come in whenever we are open.
準備中 is the opposite sign, meaning 'preparing to open' or 'not yet open'.
Compound of 営業 (business, sales) + 中 (in the state of). The reading is standard on'yomi.