Before the first sprint starts, the demo dates for the whole engagement are in the calendar. They do not move.
What a fixed date does
It forces the work to be sliced into things that can be shown. A team that must demonstrate something every two weeks cannot spend six weeks on infrastructure with nothing visible, which is the most reliable way for a project to go quietly wrong.
The rule about bad demos
If a flow is broken on demo day, we show it broken. This is uncomfortable exactly once. After that, everyone understands the meeting is for information rather than reassurance, and problems arrive early enough to be cheap.
A demo postponed to look better is a delay you find out about in month three.
Who should be in the room
Someone who will use the software, not only someone who commissioned it. The gap between those two people's reactions is the most valuable signal in the meeting.
What we bring
The build, on a real environment, with real-shaped data. Not slides. Not a recording. The demo is the software or it is not a demo.