When does a business start trading for CGT rules

[South Lakes Lad]

Hi
We rolled over our gains into a new self catering venture which we started to build in August 2016 and opened in December 2017. We registered for VAT in 2016 and took our first booking in August 2017 . We paid the VAT on that booking on our VAT return for that quarter Does our trading start date commence from that first paid deposit or from the date the guest arrived. We had numerous early bookings in the run up to December 2017 all of which we paid the VAT on. Any advice would be great as we are really ready to retire after a very stressful few years