They didn't start, or stop. It's always been an apostolic practice, alien to those who don't actually have or worse rejected the apostolic succession.
It's in the didache, a first century prescriptive manual.


14 Then came to him the disciples of John, saying, Why do we and the Pharisees fast oft, but thy disciples fast not?

15 And Jesus said unto them, Can the children of the bridechamber mourn, as long as the bridegroom is with them? but the days will come, when the bridegroom shall be taken from them, and then shall they fast.


This means the time after the ascension, when his earthly ministry ends and that of the apostolic church begins. The church age, so called, those are the days he's talking about.

>>18404750
Yeah but on Friday?
And fish is or isn't allowed in the place of meat?
There's so much cultural baggage built into the practice

>>18404752
I believe even fish and oil were totally restricted during Lent (people didn't eat much other than oatmeal and the like) but fine on fridays during the rest of the year despite meat being restricted on all fridays. Seems this article confirms.

https://sspx.org/en/news/think-lent-tough-take-look-medieval-lenten-practices-18628

>Today, only the Eastern Christian churches (many of which are not in communion with Rome) practice austerity during Lent, albeit unevenly. For instance, meat, fish, dairy, and oil are generally prohibited during the Lenten season

>the rules for Lenten penance as described by St. Thomas Aquinas:

Ash Wednesday and Good Friday were “black fasts.” These consisted of taking only one meal per day of bread, water, and herbs, after sunset.
Other days of Lent: no food until 3pm, the hour of Our Lord’s death. Water was allowed, and as was the case for the time due to sanitary concerns, watered-down beer and wine. After the advent of tea and coffee, these beverages were permitted.
No animal meats or fats.
No eggs.
No dairy products (lacticinia) – that is, eggs, milk, cheese, cream, butter, etc.
Sundays were days of less liturgical discipline, but the fasting rules above remained.

>Essentially, medieval Western Christians subsisted on bread, vegetables and some salt during Lent. Fish was permitted, though uncommon. This was consistent with the desire of the Church for its faithful to refrain from flesh meat (St. Thomas equates Our Lord giving up His Flesh for us) and to strive for greater control over our own bodies, with abstinence from the marriage act as an additional form of self-mortification.

This general prohibition of eggs and milk during Lent is perpetuated in the common custom of blessing or making gifts of eggs at Easter, and in the English usage of eating pancakes on Shrove Tuesday – a way to use up the eggs and milk before the Lenten fast. Hence the colloquial term still used by some today of “Pancake Tuesday.”