Thanks to Richard Pringle for this report:

An outdoors lunch last August in Wales was almost marred by wasps, which seemed to be attracted by the fruity odours of juice drinks and jam.

A quick bit of research (for Europe) tells me ‘Wasps are most aggressive from August to October’;  ‘Male wasps don’t have a stinger’;  ‘Females have the stinger and it is also their sex organ’; ‘Wasps become inactive at night’.