Plan a celebration people enjoy

You're hosting something and the guest list doesn't all know each other.

Help me plan this celebration so it works as an event, not just a room with food in it.

1. Tell me what the event is for in one line, and what would count as it having gone well. Everything else follows from that.
2. Build a rough timeline with the shape of the evening: arrival, the moment when everyone is present, the peak, and the point where it should end. Say when food and any speech should happen.
3. Handle the social problem: who will not know anyone, and what specifically will fix that in the first twenty minutes.
4. List what must be decided in advance versus what can be sorted on the day.
5. Tell me the three most likely ways this goes wrong and the cheap insurance against each.

Be practical about the guest of honour's tolerance for attention. Do not plan a surprise unless I have said they would like one.

Occasion, guest count and venue:
Budget and constraints:

Occasions and group gifts. Milestones, weddings and new babies, celebrations, and organising several givers or several presents.