Question: If Guānshìyīn (Avalokiteśvara) Bodhisattva (观世音菩萨) is still a Bodhisattva as named, why do Buddhists commemorate her/his day of accomplishing the path to Buddhahood (成道日) on the 19th day of the 6th lunar month (农历六月十九)?
Answer: According to the ‘Sūtra On Compassion’s Essence’《悲华经》, Guānyīn Bodhisattva is already an ancient Buddha (古佛), who has ‘since a long time ago accomplished Buddhahood’ (久已成佛), named ‘Right Dharma’s Brightness Thus Come One’ (正法明如来).
Question: Why is s/he still addressed as a Bodhisattva then?
Answer: Out of perfect compassion (大悲), s/he re-manifests as a Great Bodhisattva (大菩萨) to guide more beings to Buddhahood, and to exemplify practice towards Buddhahood. The greatest Bodhisattvas are by default ancient Buddhas, which is how they can be perfect examples of the virtues they personify and express.
Question: So s/he is both a Bodhisattva and Buddha?
Answer: S/he is technically a Buddha, manifesting as a Bodhisattva at the same time. As only Buddhas have perfect great compassion, s/he is a Buddha too. As all Buddhas are perfect (圆满) Bodhisattvas, s/he is a Bodhisattva too. Thus, s/he is both a Buddha and Bodhisattva.
Question: As Bodhisattvas vow to guide all beings to Buddhahood before they attain it, with so many beings still as non-Buddhas, how are there Buddhas in the first place, such as Right Dharma’s Brightness Thus Come One?
Answer: Indeed, Bodhisattvas strive to guide all beings to Buddhahood, while personally striving towards Buddhahood, to become the most effective Bodhisattvas. Thus, inevitably, while guiding beings, they will attain Buddhahood. Yet, to not go against their vow, they re-manifest as Bodhisattvas too, to continue fulfilling their vow to guide all other beings.