Ah okay. That's a bit more complicated, have that on my todo-list for a while (really really old code there), currently you would need to change the code for the generation algorithm.
administrator/components/com_matukio/helpers/util_events.php
Line 31 - function createNewEventNumber
At line 78 you see the return - this one needs to be modified. You don't have the event in this function.
return date('Y-m-d');
That would return the current date - but not the event begin.. The event name / begin is not even populated when this number is generated (when you create a new event), so probably the easiest solution would be to change it during saving :/
If you want i help you with that.
Yves