Whereas "false stories" can be told anywhere and at any time, myths must not be recited except during a period of sacred time (usually in autumn or winter, and only at night).... This custom has survived even among peoples who have passed beyond the archaic stage of culture. Among the Turco-Mongols and the Tibetans the epic songs of the Gesar cycle can be recited only at night and in winter.

Myth and Reality (1963)