Important note
Servy has very correctly pointed out that we cannot rule out the possibility that the event in question is owned by a third party and it implements the accessors itself. If that is the case then all bets are off.
The original answer below silently assumes that we are talking about a bog-standard, totally uninteresting event implementation.
Original answer
Events are implemented through delegates.
From MSDN:
A useful property of delegate objects is that multiple objects can be assigned to one delegate instance by using the + operator. The multicast delegate contains a list of the assigned delegates. When the multicast delegate is called, it invokes the delegates in the list, in order.
So the handlers are invoked in the order that you add them to the event.