Thousands of people are expecting to shuffle in a moving queue for hours to pay their final respects to Queen Elizabeth II when her body arrives at the Palace of Westminster in London.
Operation Unicorn is transporting the Queen's body from Balmoral to Westminster where the royal family will attend a service on Wednesday afternoon, local time.
The Queen's closed coffin will then be in Westminster Hall where members of the public will be able to visit, for four days until the morning of the state funeral.
In guidelines published on Monday, the UK government has detailed what people can and can't do when saying goodbye to Her Majesty.
A very long time.
The Queen's Lying-in-State queue will not be for the faint-hearted, with the UK government on Monday warning people could be standing in a moving line "possibly overnight".
The guidelines advised there would be very little opportunity to sit down and asked people to consider the hours-long wait before queuing or bringing children.
Step-free and accessible options will be available for those wanting to catch a glimpse of the Queen's coffin.
Each person is allowed to bring one small bag, with one small opening or zip, into the Palace of Westminster.
People should bring suitable clothing for cold or wet weather, as well as sunscreen.
There will be few options to grab a bite along the route, so snacks and drinks for the queue are a no-brainer - but they'll need to be eaten or thrown out before entering the palace.
People should also pack any essential medicine they might need for an overnight trip, and a power bank if they want to charge their phone.
There's a long list of items that aren't permitted in the palace and it starts with any bag larger than 40cm by 30cm by 20cm in size.
A security search point will be confiscating the following items while illegal items will be dealt with by police.
Essentially, people have been asked to pack manners fit for a Queen.
The UK government has urged visitors to "respect the dignity" of the event, listing rules around conduct and behaviour.
The first rule is to dress appropriately for the occasion.
"Do not wear clothes with political or offensive slogans," the guidelines state.
People have also been asked to turn their phones to silent, or turn them off.
The other three rules are pretty straight forward - keep the noise down, use the bins provided and follow the instructions of stewards and police.
Well, you can't do it "for the Gram" because taking selfies is strictly off-limits.
The first "must not" rule includes filming, photographing or using devices in the security area or the Palace of Westminster.
Anyone planning on bringing a tent or gazebos, or lighting up a barbecue, needs to scratch that off the list too.
Those wanting to pay respects to the Queen will need to do the hard yards themselves because queuing on behalf of someone else is not allowed. A wristband system will help enforce this rule.
And last but not least - don't leave personal items unattended.
The UK government issued a pretty stern warning on this one.
"If you leave bags or other items unattended they will be removed and may be destroyed," it said.