Lord Caitanya Mahaprabhu and His great Sankirtan Movement, and Lord Krsna as His original Vrindavana youthful Form, both come just "once" in a day/night of Lord Brahma.

But their appearance is only during the "day-light period" on this earth planet, which is "once" every 8 billion 640 million human years (a day of Brahma).

This means the day-light time lasts for 4 billion 320 million human years when all 1000 Maha-yugas manifest. 

The night-time lasts for 4 billion 320 million human years when Brahma sleeps, a partial annihilation happens, and no Maha-yugas manifest. 

There are 4 yugas to a maha-yuga all in human years-

1 - Satya-yuga - The "Golden Age" of spiritual development, truthfulness (Satya), and righteousness (Dharma), with people living in perfect harmony, long lifespans of up to 100,000 years - This age lasts for 1 million 728 thousand years. 

2 - Treta-yuga - The "Silver Age" characterized by a significant, but lesser, degree of virtue (dharma) than the Satya Yuga, marked by the rise of duty, ritual (sacrifices), and structured society, with the Dharma bull standing on three legs (truth, compassion, austerity) as morality begins its gradual decline into duality, famously featuring Lord Rama's era with human life spans up to 10,000 years - This age lasts for 1 million 296 thousand years.

3 - Dvapara-yuga - The "Bronze Age," characterized by a balance of virtue and vice, with dharma reduced by half, leading to increased rivalry, ego, and focus on material pursuits, rituals (temple worship), and the division of Vedic knowledge into four parts. It's a time of technological advancement (like energy understanding), growing confusion, disease, but also significant spiritual figures like Krishna in His original Vrindavana form appears, but only once in a day of Brahma. The average human lifespan is about 1,000 years with a strong focus on fulfilling scriptural duties - This age lasts for 864 thousand years.

4 - Kali-yuga - The “Age of Darkness” described in Vedic texts found in the Mahabharata and Srimad Bhagavatam. It is as a time of moral decline, spiritual confusion, and social disorder dominated by hypocrisy, cheating and quarrel. The maximum human lifespan is generally stated as 100 years, a significant decrease from previous ages like Dvapara-yuga (1,000 years), Treta-yuga (10,000 years) and Satya-yuga (100,000 years), reflecting a decline in virtue, wisdom, and physical strength; however, as Kali-yuga progresses, this lifespan is predicted to further greatly shorten, with people living much less by its end - This age lasts for 432 thousand years.

A Maha-yuga is also called a Catur-yuga and Divya-yuga.

Lord Caitanya Mahaprabhu, and Lord Krsna as His original Vrindavana Form appear in this material world very, very rarely, they only appear in one Dvāpara-yuga, and one Kali-yuga out of every thousand. 

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

Many take it for granted without realizing how rare Lord Caitanya and Lord Krsna's appearance actually is.

As said above, this means Lord Krsna and Lord Caitanya come "once" out of every 1000 Maha-yugas. 

All the 1000 Maha-yugas occur only in the 12 hour day-time period of Brahma's 24 hour day (during sunrise to sunset)

Therefore as said above, 4 billion 320 million human years equals the 12 hour "day-time period of Brahma" when all the 1000 Maha-yugas appear.

At night while Brahma sleeps there are no Maha-yugas but instead a partial annihilation of the material universe. This "night of Brahma" is when a partial annihilation occurs, that takes another 4 billion 320 million human years. 

So a full 24 hour day of Brahma equals 8 billion 640 million human years.

Lord Krsna as His original Vrindavana Form appears at the end of just "one" Dvapara-yuga out of every 1000.

And Lord Caitanya Mahāprabhu appears near the beginning of "one" Kali-yuga with His great Sankirtan Movement also out of every 1000 Kali-yugas, just after Krsna's appearance.  

So both Lord Krsna and Lord Caitanya Mahaprabhu appear just "once" during Brahma's 24 hour day-night period which is once every 8 billion 640 million human years.

So a day-time period from "sun-rise to sun-set" is made up of 1000 Maha-yugas and is called a kalpa. 

And a night-time period from "sun-set to sun-rise", where no Maha-yugas exist, when a partial annihilation of the material universe occurs while Brahma sleeps, is also called a kalpa.

There are "two kalpas" in a 24 hour day/night of Brahma.

Caitanya Caritamrta – "At the end of the Dvāpara-yuga of the twenty-eighth divya-yuga (Maha-yuga), Lord Kṛṣṇa appears on earth with the full paraphernalia of His eternal Vraja-dhāma." (CC Adi 3.10 Text 10)

Srila Prabhupada's Purport - "Now is the term of Vaivasvata Manu, during which Lord Caitanya appears. First Lord Kṛṣṇa appears at the close of the Dvāpara-yuga of the twenty-eighth divya-yuga, and then Lord Caitanya appears in the Kali-yuga of the same divya-yuga. Lord Kṛṣṇa and Lord Caitanya appear once in each day of Brahmā, or once in fourteen manv-antaras, each of seventy-one divya-yugas in duration. From the beginning of Brahmā’s day of 4,320,000,000 years, six Manus appear and disappear before Lord Kṛṣṇa appears. Thus 1,975,320,000 years of the day of Lord Brahmā elapse before the appearance of Lord Kṛṣṇa. This is an astronomical calculation according to solar years." (CC Adi 3.10 Text 10 Purport)

Brahmā’s day-time period with all 1000 Maha-yugas begins at sun-rise and lasts for 4 billion 320 million "human years" until sun-set in Brahma's day. Then Brahma's night begins when he sleeps and has no Maha-yugas, also lasts for 4 billion 320 million human years until sun-rise the next day of Brahma. 

Devotee - "Śrīla Prabhupāda, does Lord Caitanya Mahāprabhu also appear every day of Brahma?"

Srila Prabhupāda - "Yes, following Kṛṣṇa. Kṛṣṇa comes in the Dvāpara-yuga. There are four periods of each yuga: Satva, Tretā, Dvāpara, Kali. So Kṛṣṇa comes at the end of Dvāpara-yuga, and Caitanya Mahāprabhu comes in the Kali-yuga." (Lecture, SB Canto 6 Ch 1 Text 3 Melb, May 22, 1975)

So, as explained above, there are no Maha-yugas in Brahma's 12 hour "night-time" period that takes 4 billion 320 million human years to pass (until the next day's sun-rise).

When Brahma sleeps a partial annihilation of the material universe takes place.÷×.