The garden gets better in September, and almost nobody uses it
Everybody piles into the garden in July when it is 28 degrees and there is nowhere shaded to sit. Then September arrives, the light goes gold at seven instead of nine, the midges calm down, and the whole space becomes genuinely lovely. And that is exactly when people stop using it.
Late summer evening dining is the trend I would push hardest on. Slower, darker, a bit more romantic, and it requires almost nothing you do not already own.
Swap the palette before you swap anything else
Summer party styling is bright. Coral, turquoise, yellow, lots of white. None of that works at eight in the evening under low light. It goes washed out and slightly sad.
Move to deep colours. Forest and olive greens, plum, rust, dark burgundy. Then let the lighting be the only bright thing in the space. The contrast is what makes it feel atmospheric rather than just dim.
For flowers, go moodier and looser. Dahlias, hydrangeas going over, seed heads, trailing foliage. Late-season flowers are cheaper and they suit this better than anything from a florist summer bucket.
Fairy lights, done so they do not look like a bedroom
Almost everyone does string lights wrong, and it is always the same mistake. They run them round the edge of the fence at head height. That lights the boundary of your garden, which is the least interesting part of it, and it puts a glare in everyone eyeline.
Run them overhead instead. Across the space, above the table, low enough that they are in the frame when you look at people. If you have got trees, weave them through the branches so the light comes down through the leaves. Warm white, steady, not flashing.
Candles do the rest, if you can keep them lit
Nothing beats a real flame on an outdoor table. Nothing gets blown out faster either. Hurricane lanterns and covered holders are the answer. The glass shields the flame from the breeze and it throws light sideways across the table rather than straight up into everyone face.
Have a few battery candles further out in the borders too. Out at the edge of the garden nobody can tell the difference, and they will still be going at midnight when the real ones have drowned in their own wax.
The thing that keeps people outside
Here is what actually ends a September garden dinner. Not rain. Cold. Around half nine the temperature drops several degrees in about twenty minutes and somebody says the word inside. Once one person has gone in, you have lost the table.
A patio heater buys you two hours. A pile of blankets on the back of the chairs buys you almost as long and costs a fraction as much. Put the blankets out before anyone arrives, because asking is the bit that stops people using them.
Food that suits the temperature
Stop serving salads. It is the automatic summer default and it stops working the second the air cools. Think warm and slow instead. Something braised, a tray of roast vegetables, bread and butter on the table. The kind of food people linger over.
Have a plan B and do not announce it
Sort the indoor fallback quietly. Clear the dining table, know where the extra chairs are. If it holds off, nobody ever knew you had a plan. If it does not, you move in ten minutes without the party stalling.