Srila Prabhupada – ‘’ Lord Kṛṣṇa and Lord Caitanya appear "once" in each day of Brahmā (once ever 8 billion 640 million human years)’’ CC Adi 3.10 Text 10 Purport.

Srila Prabhupada – ‘’ Lord Kṛṣṇa and Lord Caitanya appear once in each day of Brahmā’’ CC Adi 3.10 Text 10 Purport.

A very rare event because this means they both come just ‘’once’’ in a 24 hour day of Lord Brahma. This means Lord Krishna and Lord Caitanya Mahaprabhu come to just "one" Maha-yuga out of every 1000.

All Maha-yugas happen during the ''day-time hours'' (between Sunrise to Sunset) of Lord Brahma's day.

So this means Lord Brahma's day from Sunrise to Sunset  equals 1000 Maha yugas.

A Maha-yugas is also known as a Catur-yuga and Divya-yuga which are -

Satya Yuga is 1,728,000 human years,
Treta Yuga is 1,296,000 human years long,
Dwapara Yuga 864,000 human years
Kali Yuga 432,000 human years.

Night-time of of Brahma's day is from ''Sunset to Sunrise'' when a partial annihilation takes place while Lord Brahma sleeps. There are also no Maha yugas during this night-time period.

So a ''day and night 24 hour period'' of Lord Brahma equals 8 billion 640 million human years.

Lord Caitanya and Lord Krishna's appearance is very rare as they both come just ''once'' over that enormous period of time.

Lord Krishna in His original Vrindavan Form, comes at the end of just "one" Dvapara-yuga out of every 1000.

Lord Caitanya Mahaprabhu comes just after the beginning of just "one" Kali-yuga out of every 1000.