You have a liability to register either when your turnover in the last 12 months exceeds £85,000 (threshold as at todays date in July 2020), or you have reason to believe that your turnover in the next 30 days alone will be £85,000 or more. If you are looking at the past 12 months it’s always on a rolling basis, so not based on a tax or accouting year, e.g. at the end of June xx you’d look at turnover from 1st July yy through to 30th June xx, and in November xx you’d look at 1st December yy to 30th Novebmer xx etc. etc.
If your past turnover exceeds the threshold then you have 30 days to inform HMRC from the date you exceed and you will need to be registered and start charging VAT by the first day of the month following e.g. if you exceed the threshold in June 2020 then you have until the end of July to let HMRC know and you would be registered from 1st August.
If you expect that you will turnover a full £85,000 in the next 30 days alone then you need to inform HMRC and be registered from the date you realise this. So if on the 1st of July you were signing a contact which would give you £50K on 7th July and £50K on 21st July then you’d register for VAT from the 1st of July and need to account for VAT on that £100K you were about to rake in.
You do not mix the two methods though, so if on 1st July you knew you’d taken in £80k in the last 11 month and that you were signing a contract which would pay you £10k during July then you don’t need to register from 1st July as you’ll only be taking £10k in the next 30 days, but once you reach the end of July then looking back you’ll have taken £90k over the last 12 months, exceeding the threshold, and so would need to inform HMRC by the end of August and be registered from the 1st of September.