Day 13 covers all of Chapter 8, The Assurance of Future Buddhahood of the Five Hundred Disciples.

Pūrṇa, the son of Maitrāyaṇī, is the subject of the opening. Of note: He praises the Buddha without asking anything in return.

‘The World-Honored One is extraordinary. What he does is exceptional. He expounds the Dharma with expedients by his insight according to the various natures of all living beings of the world, and saves them from various attachments. The merits of the Buddha are beyond the expression of our words. Only the Buddha, only the World-Honored One, knows the wishes we have deep in our minds.’

Pūrṇa, explains the Buddha, is the most excellent expounder of the Dharma: “No one except the Tathāgata excels him in eloquence.”

He did the work of the Buddha, that is, taught all living beings so that the world of the Buddha might be purified.

And in gāthās:

He obtained great supernatural powers
And the four kinds of unhindered eloquence.
Seeing who was clever, and who was dull,
He always expounded the Pure Dharma.

He expounded the Dharma of the Great Vehicle
To hundreds of thousands of millions of living beings,
And caused them to dwell in the Dharma
So that the worlds of those Buddhas might be purified.

Eventually, after protecting and expounding the Dharma of innumerable Buddhas, “He will perform the Way of Bodhisattvas step by step for innumerable, asaṃkhya kalpas, and then attain Anuttara-samyak-saṃbodhi in this world.”

In contrast to Pūrṇa’s joy, the twelve hundred Arhats thought: “How glad we shall be if we are assured of our future Buddhahood by the World-Honored One just as the other great disciples were!”

They understand that the teaching wasn’t withheld; they just did not remember that they carried that lesson. They offer The Parable of the Pricess Gem.

You, the World-Honored One, saw that the aspiration for the knowledge of all things was still latent in our minds; therefore, you awakened us, saying, ‘Bhikṣus! What you had attained was not perfect extinction. I caused you to plant the good root of Buddhahood a long time ago. [You have forgotten this; therefore,] I expounded the teaching of Nirvāṇa as an expedient. You thought that you had attained true extinction when you attained the Nirvāṇa [which I taught you as an expedient].’

“World-Honored One! Now we see that we are Bodhisattvas in reality, and that we are assured of our future attainment of Anuttara-samyak-saṃbodhi. Therefore, we have the greatest joy that we have ever had.”

(2013-02-01). The Lotus Sutra: The Sutra of the Lotus Flower of the Wonderful Dharma (Kindle Locations 4277-4282). Nichiren Buddhist International Center. Kindle Edition.