I wasn't planning on starting 5:2 until next week but got through a surprising amount of this morning without food so am wondering whether to start today.
I ate at 4pm yesterday; went through until 10.45 this morning (so over 16 hours) when I had a banana & have just had a jacket potato & salad for lunch. By my reckoning, I have about another 100 calories to go for it to be a fast day. Now, I can easily get through to 4pm only eating another 100 cals but doubt I can get through until midnight/tomorrow morning so I thought that if I did 4pm - 4pm, I'd be OK. But then it occurred to me that most people who have Monday as their fast day would probably only eat 500cals between, say, 8pm on Sunday evening and 7am on Tuesday morning making it a 35 - 36 hour fast as opposed to a 24 hour one.
I'm no expert but you broke the fast by having the banana. You could do a 16:8 day though. I think the main thing with 5:2 is to fast for as long as possible before consuming solid food to give your body maximum health benefits. I usually break my fast at 24 hours then go through to lunch the next day (a further 18 hours) and I've lost really quickly and easily but like I say, I'm new and no expert.
500 cals per calendar day, so effectively it ends up being over a 35-36 hour period.
So Sunday eat normally (but most people finish eating by about 8pm so that adds on 4 hours) Monday = 500 cals Tuesday eat normally (but as most people don't eat breakfast until about 7am that adds on another 7 hours) so 24+4+7=35 hours.
You can of course just do 24 hours if you like, but you won't get the same weight loss.