test.swift

// two-dimensional array
// should be your Event object...  mArray = [[Event]]()
var mArray = [[String]]()

// simulate your allEvents array
var allEvents = ["a 1", "a 2", "a 3", "b 1", "b 2", "c 1", "c 2", "d 1", "d 2"]

// get first "Day"
// yours will be something like:  currentDay = getDateFrom(allEvents[0])
var currentDay = "\(allEvents[0].characters.first)"

// init empty array
// yours will be Event objects...  currentEvents = [Event]()
var currentEvents = [String]()

for event in allEvents {

	// yours will be something like:   thisDay = getDateFrom(event)
	var thisDay = "\(event.characters.first)"
	
	if thisDay != currentDay || event == allEvents.last {
		
		mArray.append(currentEvents)
		
		currentDay = thisDay
		
		// reset currentEvents array:  currentEvents = [Event]()
		currentEvents = [String]()
		
	}
	
	currentEvents.append(event)
	
}

print(mArray)