I guess from a purely legalistic viewpoint, it has to be 31-1-20.
Mrs NJ and I - most unexpectantly - celebrated on 24th June ‘16 (my local Waitrose had almost run out of assorted alcoholic fizz by 10am). However, neither we nor our fellow celebrants envisaged the guerrilla warfare that would be waged in the following three and a half years as the Great and the Good tried to stymie all of us 17.4m ignorant racists who had conspired to shatter their EUtopian dream.
The real celebrations will not be justified for a while yet. We have this year to go when the shenanigans and hostilities will be resumed over the “trade deal” and during which time not much will change. I think we can all look forward to "Project Fear Mk III", as dire warnings of the pestilence, war, famine and death that will be visited upon the country unless we accede to the EU’s every demand will once again be trotted out by the usual suspects. Thankfully many of them were been consigned to the political dustbin last month.
Those celebrations may also be gradual as the country comes to reap the benefits of opening its eyes to trade and commerce with the wider world and is unbound by the over-regulated, protectionist racket that is the EU. Hopefully that wretched organisation will open its eyes too and realise that 93% of the world’s population lives outside its borders and that most of global economic growth is to be found in those places. They may also learn that keeping the UK onside will mean forfeiting some of the rights it has taken as almost a divine given. But I’ll not hold my breath.
Meanwhile, Mrs NJ and I will be sharing another bottle of bubbly on 31st January, saving some of it to chink glasses as the clock strikes eleven. For us it marks the beginning of the end of a disastrous political experiment for the UK. It was an experiment which we vowed we would vote to reverse if ever given the chance. We were finally given that chance and we grabbed it.