The great Sankirtan Movement of Lord Caitanya Mahaprabhu is a great gift to the entire universe, His appearance is very, very rare as He comes only to one Kali-yuga out of every 1000 Kali-yugas, which is once every 8 billion 640 million human years.

Lord Caitanya Mahāprabhu comes "once" in a day of Lord Brahma who lives for 100 years, which is 311 trillion, 40 billion human years.

Lord Caitanya Mahāprabhu's rare appearance is just after Lord Krsna appears as His original childhood form, who also comes to this planet "once" in a day of Brahma.

In Kṛṣṇa Lila (pastimes) Radha and Krsna combined in one spiritual Body, is Lord Caitanya Mahāprabhu. 

Many take it for granted without realizing how rare both Lord Krsna in His original childhood Form appearing at the end of a Dvapara-yuga, and Lord Caitanya's appearance, just after Lord Krsna at the beginning of Kali-yuga (1486 A.D) [Christian calendar]

Both Lord Kṛṣṇa as His original childhood Vrindavana Form, and Lord Caitanya Mahāprabhu, come only once in every ''day/night'' of Lord Brahmā during Brahma's day-time. 

Lord Caitanya appears in one Kali-yuga out of every thousand Kali-yugas. This happens during Brahma's "day-time hours" from sunrise to sunset.

During "night-time hours" from sunset to sunrise, none of the 4 yugas (Maha-yuga) exist due to a partial annihilation happening at night and Brahma sleeping.

This takes 4 billion 320 million human years, the length of Brahma's night time period, but also the length of Brahma's day-time as well.

There are 4 yugas to a Maha-yuga or Catur-yuga. As said above, Lord Caitanya appears to one Kali-yuga out of every 1000 Kali-yugas with great His Sankirtan Movement.

And Lord Krsna as His original Form also only appears once at the end of a Dvapara-yuga, in one Dvapara-yuga out of every 1000 Dvapara-yugas just before Lord Caitanya Mahāprabhu's appearance at the beginning of Kali-yuga.

There are 4,320,000 human years in one maha-yuga (that is made up of 4 individual yugas), multiply that by 1000 and it will equal Brahma's 12 hour "day-time" period from sunrise to sunset, which is 4 billion 320 million human years long.

Then add another 4 billion 320 million years for Brahma's 12 hour night-time period (sunset to sunrise), when a partial annihilation, with no yugas, takes place and Brahma sleeps. 

Therefore the "day-time" hours and "night-time" hours together, adds up to 8 billion 640 million human years which is a 24 hour ''day and night'' of Lord Brahma.

A day/night also equals two Kalpas, one for daytime, one for nightime.

Therefore, we can understand that both Lord Krsna in His ''original'' Form, and Lord Caitanya Mahāprabhu's appearance, are very, very rare as they both only come once over that enormous period of time.

Keep in mind, modern mundane science foolishly claim the entire universe began 13.5 billion years ago from a nonsense so called big bang. They have no idea what's going on.

Yes, Krsna comes once every 1000 Maha-yugas in a Dvapara-yuga,  and Lord Caitanya Mahāprabhu comes once in Kali-yuga, which all happen during the day-time period of Brahma. 

This also means there are 999 Kali-yugas that have NO Lord Caitanya Mahāprabhu or His Golden Age of Sankirtan in it, this only happens to one Kali-yuga in a day of Brahma and 999 Dvapara-yugas where Kṛṣṇa as His original childhood Form does not appear. 

As explained above, Brahma's night from "sunset to sunrise," is when a partial annihilation takes place. This is also called the ‘Night of Brahma’ where their are no Maha-yugas during Brahma's night-time. 

So, together a 24 hour ''day/night'' of Brahma equals 8 billion 640 million human years.

Also in the rare Kali-yuga Lord Caitanya Mahāprabhu appears in, there is a 10,000 year Golden Age known as the Sankirtan Movement of Lord Caitanya Mahaprabhu, which is the congregational chanting of the Hare Krsna Maha mantra in every Town, Village and City on the planet. 

In this very rare Kali-yuga we are all very fortunate if we take up the great Sankirtan Movement of Lord Caitanya Mahaprabhu and spread the chanting of Hare Krsna allows over the globe.