A minute's silence will be held across the UK to remember the 38 people, including 30 Britons, killed in the Tunisia beach attack a week ago.
Flags will be flown at half-mast over Whitehall and Buckingham Palace, while play at Wimbledon will be delayed.
The Queen and Prime Minister David Cameron will join the midday silence.
The first inquests into the deaths of the Britons will begin later, with the bodies of more of the dead expected to arrive back at the Oxfordshire Royal Air Force Brize Norton.
The foreign secretary has said all 30 British people killed have been identified. Philip Hammond said he was confident the figure was the final British death toll from the beach shootings in Sousse last Friday.
The Queen and Duke of Edinburgh will join staff at the University of Strathclyde marking the silence, while Mr Cameron will be in his Witney constituency in Oxfordshire.
A number of mosques are excepted to participate in the silence, and many will also remember the victims during Friday prayers.
Police officers across the country will take part, the National Police Chiefs Council said.
And a special ceremony will also be held at the scene of the killings in Sousse, where dignitaries and tourists are expected to attend.
At Wimbledon, matches on the outdoor courts usually start at 11:30 but will begin at 12:15 to allow spectators and participants to take part in the silence.
Later, the first inquests will open at West London Coroner's Court. Coroner Chinyere Inyama is expected to open and adjourn the hearings.
Further inquests are due to be opened at the court on Saturday and Sunday.
Post-mortem examinations will be carried out before the bodies are released to their families.
The bodies of 17 of the British victims have now returned to the UK.
The repatriation of the dead is likely to take several days, with two further flights planned for Friday and Saturday.
Travel companies Thomson and First Choice said all 30 British people killed were its customers.
"The whole company would like to extend our deepest sympathies to the family and friends of those involved in this tragic event," it added.
"Our main focus now is to ensure the families of the deceased and our customers who have been injured receive all possible support at this incredibly difficult time."
Tunisian authorities have identified 28-year-old student Seifeddine Rezgui as the gunman who carried out the attack.
They are also holding eight suspects in custody on suspicion of being directly linked to the attack, which Islamic State has claimed responsibility for. Four others who were held have been released.
On Thursday, UK Defence Secretary Michael Fallon set out the case for air strikes on IS targets in Syria in Parliament on Thursday.
He has suggested the Tunisia attack may have been planned by IS in Syria.
Meanwhile, UK authorities are investigating the Tunisia tragedy.
The Metropolitan Police said more than 160 officers were interviewing witnesses to the attack who had returned to the UK.
A total of 20 officers have been sent to Tunisia by the Met's Counter Terrorism Command, which is leading the coroner's investigation.
The National Policing Counter Terrorism Headquarters has also sent specialist security advisers to Tunisia, to support a review of security at resorts and tourist attractions.
Scotland Yard has previously said its investigation into the attack is likely to be one of the largest counter-terrorism deployments since the London 7/7 bombings in 2005, which killed 52.