The åtba (taken from the Spanish word alba, meaning "daybreak") is the first church bell of the day.
Before the war, it was more accurate to say, the first church bell of the still-dark, pre-dawn, because it was rung at 4AM! The first Mass of the day was at 430AM.
One sister remembers the bells being rung at 4AM and she said you could hear the bell of the Cathedral in San Ignacio clear down to Santa Cruz. Everybody heard it.
Then again, a lot of people were already awake by then. The Spanish missionaries mention in their letters and reports that many Chamorros prided themselves in waking up early. You can still see this trait if you are ever zesty enough to go to the 6AM Masses that still exist today. The majority at Mass will be older Chamorros who learned this trait from their parents.
It makes sense. Who wants to farm the fields at 12 noon? So they headed for the ranch while still dark to work in the cool morning, get into the shade by mid-day and finish up when the sun was less unforgiving.