Generally you are unlikely to get a wasp sting until Autumn, unless you accidentally put your hand or foot on one and they are defending themselves, or unless you disturb a wasps nest.
Up until late July and early August they are busy bringing up and feeding larval wasps, chasing insects, and foraging for food and maintenance materials for the wasps nest.
After that their job is mainly done and they gorge themselves on the food they collect, especially on ripe and fermenting fruit; they become more and more dependent on sweet foodstuffs like these and will aggressively seek it out.
Additionally it will be getting hot and very crowded in the nest; the internal temperature of the nest is 5 – 10ºC above the outside temperature, so on a good summer day it can reach 25 ‘s 30ºC inside the nest. On top of this the nest’s population is at its highest.
It is at this time when they are most likely to sting humans, partly due to bad tempers caused by the heat and overcrowding in the nest, and partly in a semi-drunken reaction to being obstructed in their quest for sweet food.