7 day visa question

Hi, if I'm a Mainland China citizen and get a visa for a sightseeing trip to Hong Kong and it lasts a max of 7 days, what does that actually mean? Can I arrive at Hong Kong on a Saturday evening and leave the next Saturday afternoon? Or would that count as 8 days?

If it counts as 8 days, what are my options to be able to stay that amount of time since I already bought plane tickets w/o knowing this policy? thanks.