Plan an event without forgetting something

I'm putting on my mum's 70th, a lunch at ours, nothing formal for about 25, four kids under ten, two people who can't do stairs. It's six weeks away, a Saturday.

Constraints: around 300, one oven, small garden, no idea if it'll rain, my brother is doing the drinks and will forget

Plan it backwards from the day.

Give me a countdown with dated blocks: what to do six weeks out, then a month, then a fortnight, then the week before, then the day before, then the morning of. One line per job, and say roughly how long each takes.

Then two more things:
- A running order for the day itself with times, from the first guest arriving to people going home.
- What usually goes wrong at this kind of event, and what to do about each one in advance. Give me the real ones, including the awkward ones about people, not just the weather.

Work inside the constraints I gave you. If something I want isn't possible with what I've got, say so early rather than planning around it quietly. Assume I'm doing most of it myself.

What you get: A dated countdown, a running order for the day, and the things that go wrong at this sort of event.