Ember days occur four times a year in the Church calendar and it is believed that their origin lies in pre-Christian agricultural observances. The Church often “baptized” those things which the newly-converted had held to be important and gave them a Christian significance. Some of the earliest Fathers of the Church in the West (Ember Days do not appear on the Byzantine calendar) made reference to the Ember Days: Pope Callixtus I (217-222) called for these days of fasting, as did Pope Leo the Great (440-461) and Pope Gelasius I (492-496).

Source: https://www.stgregoryoc.org/article/article-archive/ember-days/