Buckingham Palace issued a statement: ‘On Sunday 5th April at 8pm (BST) Her Majesty The Queen will address the UK and the Commonwealth in a televised broadcast. As well as on television and radio, The Queen’s address will be shown on the Royal Family’s social media channels.’
https://twitter.com/RoyalFamily/status/1246061575242428416
The palace says the message was recorded at Windsor Castle and will be televised on Sunday. The Queen relocated from Buckingham Palace because of the outbreak.

The value of an address by the Queen is that it is so rare . It will mark only the fourth special address of the monarch’s 68-year reign. ‘It is a question of when, not if. The value of an address by the Queen is that it is so rare” — it’s a case of when is the optimum time to do it. The Queen wants to get it right.
https://twitter.com/talkRADIO/status/1246063764409114625?s=20
The Queen will likely praise NHS staff, show her her admiration for country’s resilience, reassure the nation through the current coronavirus crisis. Prince Charles, who addressed the nation last week, said in his speech: „As a nation, we are faced by a profoundly challenging situation, which we are only too aware threatens the livelihoods, businesses and welfare of millions of our fellow citizens. None of us can say when this will end, but end it will. Until it does, let us all try and live with hope and, with faith in ourselves and each other, look forward to better times to come.”
The speech will be broadcast on both television and radio, as well as the @RoyalFamily social media channel.
Source: Twitter @Royal_Family