I think quoting the surrounding canons mitigates the apparent contradiction:
Can. 1249 All Christ’s faithful are obliged by divine law, each in his or her own way, to do penance. However, so that all may be joined together in a certain common practice of penance, days of penance are prescribed. On these days the faithful are in a special manner to devote themselves to prayer, to engage in works of piety and charity, and to deny themselves, by fulfilling their obligations more faithfully and especially by observing the fast and abstinence which the following canons prescribe.
*Can. 1252 The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance.
Can. 1253 The Episcopal Conference can determine more particular ways in which fasting and abstinence are to be observed. In place of abstinence or fasting it can substitute, in whole or in part, other forms of penance, especially works of charity and exercises of piety.*
To answer the OP: The authority to establish the precise means of observing penance is given to the local Episcopal Conference.
However also worth noting are the following:
*Can. 1244 §1 Only the supreme ecclesiastical authority can establish, transfer or suppress holydays or days of penance which are applicable to the universal Church, without prejudice to the provisions of Can. 1246 §2.
§2 Diocesan Bishops can proclaim special holydays or days of penance for their own dioceses or territories, but only for individual occasions.
Can. 1245 Without prejudice to the right of diocesan Bishops as in Can. 87, a parish priest, in individual cases, for a just reason and in accordance with the prescriptions of the diocesan Bishop, can give a dispensation from the obligation of observing a holyday or day of penance, or commute the obligation into some other pious works. The Superior of a pontifical clerical religious institute or society of apostolic life has the same power in respect of his own subjects and of those who reside day and night in a house of the institute or society.*
Canon 1244 grants the Diocesan Bishop the authority to proclaim special days of penace, but I don’t know if he may establish the particular means of observing penance?
I Am Not A Canon Lawyer
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