So what happens on completion day?

[Viognier]

I'm finding it hard to comprehend that everyone in the chain has one day in which to move everything out of their respective houses, clean then (hopefully) and then hand over keys.

We are at the bottom of the chain and renting so have a bit of luxury time wise but I'm thinking about people up the chain moving out of one house and in to another while the people in that house are doing the same . What really happens?

The day before completion the removal company come and load the truck with almost everything, leaving you with beds and whatever you might need overnight. You eat takeaways for a few days. The next day they arrive at 8, load the remaining furniture and head to the new house. You have to wait for a call from your solicitor to say its all gone through, it's usually around lunchtime. There can be some waiting around, this is when you clean although you'll have done a lot the previous day.