As I noted, the sections of the CIC that deal with feast days and days of penance are exclusively about obligatory days. Canonically speaking, it appears that if it’s not obligatory, it’s not technically a feast day.
The section on feast days surely gives the impression that it concerns only days of precept; the section on penitential days, however, does not. After all, it includes the season of Lent among the penitential times, and that season, apart from Ash Wednesday and its Fridays, imposes no penitential obligation whatsoever upon the faithful. Furthermore, if you look at the Latin text, the section on feast days does concern only feasts of precept (
de praecepto), while the earlier canon granting the authority to proclaim special feasts in one’s own territory is not included in this section and, more importantly, does not contain the all important words
de praecepto that are annexed to every mention of a feast in the subsequent section. Thus it would appear not to pertain specifically to days of precept.
Now, that is not a slam dunk case against the authority to establish an obligation to attend Mass - it may either be contained generically or found somewhere else in the code - but when one combines the non-obligatory nature of some of the days in the section on “Penitential Days” with the lack of any reference to bishops establishing special feast days
of precept, we need to conclude that there is no
explicit grant of authority to establish such an obligation in the canons that have been presented. We need further evidence.
BTW - the fact that the code does speak of feast days that are not feasts of precept goes against the assertion that, canonically speaking, there is no such thing as a feast day not of precept.