I don't understand the timing of the visit, but it will go ahead, there will be lots of pomp, oodles of ceremony and even more security. People can and will protest (we are a free country) and international relations between the UK and USA will be girded a little more strongly than before.
About the timing, as far as I understood, the invitation was extended by Mrs. May already in 2017, but they couldn't find a common date so far.
In order to really receive him during his current term, it was about time now, and I guess the 75th anniversary of the Normandy landings (after all a common success of the US & UK) came in handy for the programme.
But I agree with those of you who said that any commemoration festivities for D-Day are not the right place to protest against Trump.
Being head of state, receiving other heads of state, no matter how "scumbaggy" they are or what she personally thinks of them, is part of QEII' job descriptions. After all, the BRF should not utter any political opinions in whatever direction (a rule Charles loves to ignore ... he'll need to learn it someday