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

Having last month concluded Chapter 16, The Duration of the Life of the Tathāgata, we begin again at the top with Śākyamuni’s call to “Understand my sincere and infallible words by faith.”

Thereupon the Buddha aid to the great multitude including Bodhisattvas and others, “Good men! Understand my sincere and infallible words by faith!”

He said to the great multitude again, “Understand my sincere and infallible words by faith!”

He said to them once again, “Understand my sincere and infallible words by faith!”

Thereupon the great multitude of Bodhisattvas, headed by Maitreya, joined their hands together and said to the Buddha, “World-Honor done, tell us! We will receive your words by faith.”

They said this three times. Then they said once again, “Tell us! We will receive your words by faith.”

The Introduction to the Lotus Sutra offers this:

Sakyamuni begins [Chapter 16, The Duration of the Life of the Tathāgata,] by appealing three times for his listeners to “understand my sincere and infallible words by faith.” To this appeal, all the Bodhisattvas headed by Maitreya responded each time, “World-Honored One, tell us! We will receive your words by faith.” Then Sakyamuni replied, “Listen to me attentively! I will reveal to you my hidden core and supernatural powers” (p. 241).

Here “hidden core” means his deepest innermost self, and “supernatural powers” are actions outflowing from that hidden core.