As contributor Augustinus remarked, and as it is well known, the obligation of fasting and abstinence on Christmas Eve has been legally (canonically) void for many decades - and this was made even clearer by the regimen of fasting and abstinence established by the current Code of Canon Law.
However, since the very first days of this blog, there has been curiosity from some readers, who for reasons of personal devotion wanted to keep this practice, on the historical legal precedent for anticipating the fasting and abstinence of Christmas Eve from December 24 to December 23. There were indults and permissions for this practice in different occasions, but the general permission dates from 1959, when Pope Blessed John XXIII extended it to "all faithful of the entire Catholic world". The decree of the Sacred Congregation of the Council, once somewhat hard to find, is now widely available thanks to the great act of the current Pontificate of making all editions of the Acta Apostolicae Sedis available online:
[Source: AAS 1959 (918); translation: Rorate]SACRA CONGREGATIO CONCILII
DECRETUM
FACULTAS ANTICIPANDI OBLIGATIONEM ABSTINENTIAE ET IEIUNII PERVIGILII NATIVITATIS D. N. IESU CHRISTI.
Plurimorum Episcoporum ex multis Nationibus votis obsecundans, Ssmus Dominus Noster Ioannes Pp. XXIII, praesenti Sacrae Congregationis Concilii Decreto, gratiam deinceps anticipandi obligationem abstinentiae et ieiunii a die vigesima quarta, pervigilio Nativitatis Domini Nostri Iesu Christi, ad diem vigesimam tertiam mensis Decembris omnibus orbis catholici fidelibus concedere dignatus est.
Datum Roma, die 3 Decembris 1959.
P. Card. CIRIACI, Praefectus
P. Palazzini, a Secretis
_______________________________________
SACRED CONGREGATION OF THE COUNCIL
DECREE
ON THE POSSIBILITY OF ANTICIPATING THE ABSTINENCE AND FASTING OF THE VIGIL OF THE NATIVITY OF OUR LORD JESUS CHRIST
Favorable to the requests of numerous Bishops of several Nations, Our Holy Lord Pope John XXIII, through the present Decree of the Sacred Congregation of the Council, has deigned to grant to all the faithful of the Catholic world henceforth the possibility of anticipating the obligation of fasting and abstinence from the twenty-fourth, Vigil of the Nativity of Our Lord Jesus Christ, to the twenty-third day of the month of December.
Given in Rome, December 3, 1959.
P. Card. CIRIACI, Prefect
P. Palazzini, Secretary