Gaius Julius Caesar Augustus Germanicus - also known as Caligula - is assassinated as part of a larger plot en route to the race track. The rest of the day is chaos, as the Praetorians and Senate attempt to agree on a new ruler for the empire.
Essentially most characters would only need to know things between 50CE through to the current time.
AeRo is mostly AU after Caligula. However, the births of a few important dates were put in place to assist in working out ages, etc.