You are being invited to a summer evening party (maybe in a garden, if you're lucky and the weather holds!), and someone says, almost casually, 'I expect we'll be having some Pimms'. Who or what is/are these?
Pimms are paid entertainers / 'animateurs/-trices' such as fortune-tellers, instant sketch portrait painters etc. who add a touch of style to the event.
Pimms is a proprietary brand of alcoholic drink, usually mixed into a 'fruit cup' with floating ice, fruit and mint sprigs.
Pimms are various forms of aerial entertainment including balloons, Chinese-style lanterns and fireworks.
Pimms are party-games such as charades, so you should come prepared to join in.