An excerpt from a letter dated around 21/Feb/06.
I have never gone into this very deeply, but something like two thousand five hundred Arahants spontaneously came to the Buddha on the full moon of February. I can only think that this being the coldest full moon of the year, and thus the most unconducive to any kind of unnecessary activity, the Arahants wanted to demonstrate the correct attitude of a Dhamma practitioner to such reluctant negativity, for the benefit of all sentient beings, for all time to come. The Lord Buddha, in his infinite wisdom and boundless compassion, complied with a sermon uniquely befitting such an auspicious occasion. The Ovaaddha Patimokha, in only three lines, encapsulates the entire Buddha Dhamma with just these few words:
Do only Good
Avoid all Evil
Purify the Heart
This was the only recorded time that the Arahants came together in quite this way.