The Dementors of Azkaban had no need for calendars.
They measured time by hunger.
Every month, three days before feeding, the Dementor Leader forbade all the others from eating on their own.
The flickers of hope the prisoners occasionally felt, the satisfaction of a mouthful of food, and the brief happiness that visited their dreams all accumulated over those three days.
On the final day, the leader descended from the highest tower and passed every cell, harvesting every emotion.
That day belonged to it alone.
The other Dementors could only trail behind and watch.
If the leader ate its fill, it might leave a little from the last few cells for those who had behaved best lately.
For some reason, this month's feast was unusually abundant.
Dementors had no noses. They could not smell the grease, salt, and roast fish that had appeared in the prison. But they could sense that prisoners who once only huddled in corners awaiting death had begun looking forward to tomorrow, to their next meal, even arguing over something golden.
The leader did not understand French Fries.
It only knew that more positive emotions than usual had taken root throughout the prison.
The three-day fast had clearly been necessary.
One of its followers had also been exceptionally obedient lately. Whenever another Dementor approached a cell, it stepped aside without competing, and it had never broken the fasting order.
Each day, it even gave up its chance to feed and let the others drink their fill.
The leader was pleased.
So when that follower came to the foot of the tower before the feast and indicated that it had found a particularly rich meal, the leader allowed it to come along.
Once I've eaten, I can reward it with a little, the leader thought.
The Dementor