Day 21 covers all of Chapter 16, The Duration of the Life of the Tathāgata.
Having last month considered how old is really, really old, we come to the expedients of the Eternal Buddha.
Good men! During this time I gave various names to myself, for instance, the Burning-Light Buddha. I also said, ‘That Buddha entered into Nirvana.’ I did all these things only as expedients.
Good men! When some people came to me, I saw the strength of the power of their faith and of the other faculties of theirs with the eyes of the Buddha. Then I named myself differently, and told them of the duration of my life differently, according to their capacities. I also said to them, ‘I shall enter into Nirvana.’ I expounded the Wonderful Dharma with these various expedients, and caused the living beings to rejoice.
Good men! When I saw that some people of little virtue and of much defilement were seeking the teachings of the Lesser Vehicle, I told them, ‘I renounced my family when I was young, and attained Anuttara-samyak-sambodhi [forty and odd years ago].’ In reality I became the Buddha in the remotest past as I previously stated. I told them this as an expedient to teach them, to lead them into the Way to Buddhahood.
Lotus World: An Illustrated Guide to the Gohonzon offers a view on this:
This view of Shakyamuni Buddha, specifically in the sixteenth chapter of the Lotus Sutra [as the original and eternal Buddha], is the key to the true nature of enlightenment according to Nichiren Buddhism.
Shinjo Suguro explains that the original and eternal Shakyamuni Buddha provides Buddhism with a united faith:
“In Buddhism, various Buddhas have been established as objects of devotion for different pious believers. Since each Buddha has a good reason for being venerated, Buddhism permits us to worship any or all of them. Nevertheless, the Most-Venerable-One should be One, just as the Truth is One. The second half of the Lotus Sutra (Hammon) emphasizes such a Buddhist position regarding the unity of faith. As the object of faith is absolute, it must relate to the realm of eternity. Generally we think of Shakyamuni as a historical figure, bound by the limitations of time and space, and only a provisional manifestation of the infinite, eternal Buddha. According to the Lotus Sutra, however, every Buddha, including the historical Shakyamuni Buddha, is a representation of the eternal original being of Shakyamuni.”
Lotus World: An Illustrated Guide to the Gohonzon