Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.

Having last month concluded the Parable of the Skillful Physician and His Sick Children, we compare Śākyamuni to the Physician.

“Good men! What do you think of this? Do you think that anyone can accuse this excellent physician of falsehood?”

“No, World-Honored One!”

The Buddha said:

“I am like the father. It is many hundreds of thousands of billions of nayutas of asaṃkhyas of kalpas since I became the Buddha. In order to save the [perverted] people, I say expediently, ‘I shall pass away.’ No one will accuse me of falsehood by the [common] law.”

The Daily Dharma from June 5, 2017, offers this:

The Buddha gives this explanation in Chapter Sixteen of the Lotus Sūtra. In the story of the Physician and his children, the father leaves and sends word that he has died when his children refuse to take the antidote he has prepared for them. He gave his children no choice but to accept what they already had and make the effort to improve themselves and set aside their deluded minds. In the same way, when we take the Buddha for granted, and close our eyes to the Wonderful Dharma he has given us, he disappears. It is only when we open our eyes and see clearly this world and ourselves in it that we can recognize the Buddha and how he is always leading us.