Satan's Little Season, also called simply "The Little Season" refers the time of the end of the 1000 year reign of Jesus Christ with his saints on the Earth. The 1,000 year reign is commonly known as the millennium, or the millennial reign of Christ.

Rev. 20:1-3, 7-8

[1] And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.
[2] And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
[3] And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.

[7] And when the thousand years are expired, Satan shall be loosed out of his prison,
[8] And shall go out to deceive the nations which are in the four quarters of the earth, Gog and Magog, to gather them together to battle: the number of whom is as the sand of the sea.

The Millennium was marked by several significant things, notably:

1) Satan was bound in the bottomless pit for 1,000 years.
2) Jesus Christ and his Saints - in particular the resurrected saints that were martyred during the great tribulation reigned with Christ on the Earth.
3) A time of worldwide prosperity and relative peace. Relative meaning there were no large-scale wars during this period. Jesus ruled with a rod of iron, meaning His law was enforced strictly and also allowing that there would be opposition to his rule but it would not be allowed to be overt. (His opposition would have to go underground which is likely the soil on which the secret societies were grown.)

See the little season eschatology page for more details.