Both Lord Krsna and Lord Caitanya Mahaprabhu appear just "once" during a 24 hour day of Brahma but only during the daylight hours of Brahma's day when the 1000 Maha-yugas occur.

Lord Caitanya and Lord Krsna’s appearance in this material world is very, very rare.

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. 

Both Lord's appear just "once" in a "24 hour day" of Brahmā, but ONLY during the "day-light hours" when the 1000 Maha-yugas also only appear.

There are no Maha-yugas during Brahma's "night-time period" because he sleeps and during his sleeping a partial annihilation of the material universe occures.

 

A "24 hours day/night" of Brahma equals 8 billion 640 million human years.

Brahma's day is divided into "day-time hours," when the 1000 Maha-yugas appear, and night-time hours when no Maha-yugas appear.

In other words, all 1000 maha-yugas ONLY occur during the "day-time period" of Brahma between "sunrise to sunset."

So, to be clear, the make up of Brahma's "24 hour day/night" period is 12 hours of "day-time" and 12 hour of "night-time," 4 billion 320 million human years is Brahma's "daytime period,"  while another 4 billion 320 million human years is also Brahma's night-time hours.

So, as said above the full 24 hour day/night period of Brahma equals 8 billion 640 million human years.

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

1 - Satya-yuga - 1 million 728 thousand years. 

2 - Treta-yuga - 1 million 296 thousand years.

3 - Dvapara-yuga - 864 thousand years.

4 - Kali-yuga - 432 thousand years.

A maha-yuga is also called a catur-yuga and divya-yuga.

Lord Krsna appears at the end of just "one" Dvapara-yuga out of every 1000 during Brahma's day-time.

And Lord Caitanya appears near the beginning of just "one" Kali-yuga with His Sankirtan movement out of every 1000 Kali-yugas.

So as said above, both Lord Krsna and Lord Caitanya Mahaprabhu appear just "once" over Brahma's 24 hour "day/night" period, but ONLY during the daylight hours.

Lord Caitanya and Lord Krsna (in His "original" Form) both appear "once" during the "day/night time" full day of Brahma.

So a ''day-time'' period made up of 1000 Maha-yugas is called a kalpa, and a ''night-time'' period of a partial annihilation of this universe is also a kalpa 

There are "two kalpas" in a "day/night" (24 hour period) of Brahma which together is 8 billion 640 million human years.

As said above, Lord Caitanya and Lord Krsna's appearance are very rare as they both come "once" over that enormous period of time as Prabhupada has explained.

Srila Prabhupada –"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 – "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 (maha-yuga), and then Lord Caitanya appears in the Kali-yuga of the same maha-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 (maha-yugas) in duration. 

From the beginning of Brahmā’s day-time period of 4 billion, 320 million human years, six Manus appear and disappear before Lord Kṛṣṇa appears. Thus 1 billion, 975 million 320 thousand years of the day of Brahmā elapse before the appearance of Lord Kṛṣṇa. This is an astronomical calculation according to solar years." (CC Adi 3.10 Text 10)

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 also takes 4 billion 320 million human years to pass) while Brahma sleeps and a partial annihilation takes place.

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

Srila Prabhupāda - "Oh. Anyway, I give you the right figure, four million, according to American or English calculation, 4,300,000 years and multiply it by one thousand. Then what it comes according to English calculation?"

Paramahaṁsa - "4,300,000?"

Srila Prabhupāda - "That is twelve hours. And add again twelve hours, night. Then eight billion...? So Kṛṣṇa comes after this period. In one day, after one day of Brahma, He appears."

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." (SB Canto 6 Ch 1 Text 3 Melb Australia May 22, 1975)

1 kalpa equals Brahma's 12 hour day-time period.

1 kalpa equals his 12 hour night-time period.

So there are 2 kalpas in a 24 hour day of Brahma that takes 8 billion 640 million human years.

The Catur-yuga is also known as Divya-yuga or Maha-yuga and is made up of the four yugas mentioned above.***.