Hi chris and welcome:

The way 5:2 works is you go to bed, get up, eat 500 or fewer calories during your waking hours, go to bed, get up and eat normally.

When you eat your 500 (or, I guess, 800) cal. during the day is up to you – research shows it makes no difference if you spread your calories out over two or three meals or just eat one in the evening.

This thread will answer most of your questions: https://thefastdiet.co.uk/forums/topic/the-basics-for-newbies-your-questions-answered/

Good Luck!