I have one hypotheses as to why they would sting on a Saturday afternoon and that is if the beekeeper works, then a Saturday afternoon would be a good time to inspect the bees if the weather is reasonably warm and fine. After June I normally only inspect my bees once in July. They require weekly inspection in the Spring and early Summer to prevent swarming. So, if the bees are normally OK and only become aggressive on a Saturday afternoon, then that would probably be the time the beekeeper took their hive apart for inspection and the bees became angry, some colonies do not like intruders! After the beekeeper had left the apairy, the guard bees probably started patrolling a wider area, to try and prevent this intruder or other intruders from returning. They are overly defensive bees.