Here in our part of Germany this happens without invitations so for birthdays - especially round ones - you have to expect half the village. Fortunately they don't all arrive at the same time and don't all stay for an inordinate amount of time.
The day before check cups, saucers, plates and glasses. Don't forget to get out serving bowls and platters for finger food, crisps, etc. They don't get used much so wash them or line them with paper serviettes. Use disposable cutlery if necessary. Wash thermos flasks thoroughly for keeping coffee and tea.
Use an old white sheet to cover a wallpapering table or garden/ camping tables that you may be setting up for a buffet.
The day before, you can bake mini sausage rolls, savoury tarts, cup cakes, but avoid anything that needs more than two steps to make on the day, such as sandwiches. Prepare spreads, cut ham, smoked salmon etc to size in advance, then on the day, slice baguettes to make small slices for open sandwiches which can quickly be assembled.
Fresh fruit goes down well cut into bite size pieces, but don't make a dip for crackers/vegetable fingers as everyone will be dipping into it.
If you have the time and inclination, dates wrapped in bacon (held with a cocktail stick) are very popular and tasty. Make up an oven tray full ready to bake as these are best served warm. Also popular, and can be made in advance if you have somewhere to keep them chilled are sticks of celery with a firm paste of blue cheese, butter, cream and seasonings, chopped chives, piped along the hollow side of the sticks with a star nozzle, then cut into bite-size lengths.
Chill drinks the day before, too. You probably have a few things in the fridge that don't need to be in there which you can take out to make room. Freeze ice packs to keep drinks in cooler bags next to the table. Decent fruit juices and water into pretty glass jugs or carafes.
Don't forget decorations. Colour coordinated serviettes, silk flowers, candles (to disguise the bedsheet!)