The Age of Aquarius

Paul Le Cour's L'Ère du Verseau (1937) is often credited with the idea. Yet Levi Dowling's Aquarian Gospel (1908) already offered 'the religion of the Aquarian Age', and its 1911 edition admitted astronomers disagreed on an age's start.

Levi H. Dowling, The Aquarian Gospel of Jesus the Christ (Los Angeles, 1908; 1911 edition, title page and introduction by Eva S. Dowling); Paul Le Cour, L'Ère du Verseau, l'avènement de Ganimède (Paris, 1937)

The astrological belief that humanity is passing from the Age of Pisces into an Age of Aquarius.

Sources2 sources

Primary source

Levi H. Dowling; introduction by Eva S. Dowling, The Aquarian Gospel of Jesus the Christ: the philosophic and practical basis of the religion of the Aquarian Age of the world, 1911, Introduction, pp. 5-6An Age lasts a little over 2,100 years; humanity stands on the Piscean-Aquarian cusp; the start date is disputed among astronomers.
The Aquarian Age is pre-eminently a spiritual age