Business days...

excuse my wording, but what the f*ck are they? I've heard that monday to friday are business days- but then what about 6 business days, do u just gloss over the weekend, or do you make saturday and sunday business days when their turn is up? Like for example: "honey i'll be gone for 8 business days, have fun!" how long are those 8 "business" days

context: i am not native, yet i have a c1 level of education with a high vocab in english (including the term business days), but of some choice things i am just completely unaware of the meaning