We have discussed this quite a lot on the main 5:2 thread. The concencus is that going 24 hours completely without food would work, but if you try to include the 500 cals into that period then yoy are not really fasting. A true 5:2 is 500cals in 36 hours not 24.
5:2 fasting days are sleep to sleep, but as Sheila mentioned, it may be possible to do a 24 hour modified fast which would be a form of intermittent fasting but not really 5:2. For more details see the links and tips thread where a recent post should answer your questions.