I always thought it would be end of the day, but I'm not sure.
I would interpret it as the time between Friday and Saturday, and I think most people would, but it's still definitely ambiguous. If you're writing or speaking, try to use an alternative instead:
- 11:59 PM on Friday
- The beginning of Saturday
- At midnight between Friday and Saturday
If someone else says it, it's not a bad idea to ask for clarification. Even though it's more common for it to mean Friday, there are still times when it will mean Saturday, and neither is totally correct or incorrect.