While the Buddha was living at Wángshè, at the full moon on the 15th day of the 3rd month, there was suddenly a meeting of all his followers at the Bamboo Forest Retreat, and it was attended by 1,250 luóhàn and dìzi. Those who were spreading the doctrine came back to report about their work. All the disciples were delighted to see each other and learn how well it had gone.
The Buddha took advantage of the occasion to teach them how to avoid evil and do good and how to put their minds at rest.
What we remember today about this convention is that all these disciples, all of whom had left their families to seek salvation and spread the word about how to avoid suffering, all of them attended, but they had not made any prior arrangement to do so. It just happened.
What the Buddha preached to them is today preserved in a text called Pōluótí Mùchā Jiàojiè 波罗提木叉教诫. And the event itself is commemorated as the "Festival of Reverencing the Law (Jìngfǎ Jié 敬法节).