By custom and former law, the liturgies of the Pre-1955 Triduum Liturgies of Holy Week had to be celebrated in the morning hours of Holy Thursday, Good Friday, and Holy Saturday. This was altered with the introduction of the Reformed Holy Week Liturgies of 1955 as well as the 1957 Apostolic Constitution Christus Dominus that reduced the Eucharistic fast to three hours, thus enabling the celebration of Mass past noon.

Amongst the changes brought about by the Pian reform in the 1950s was the change of time for Tenebrae offices, the movement of the Holy Thursday Mass of the Lord's Supper (but not the Chrism Mass) and Holy Saturday Vigil and Mass to evening hours (after sunset), and the movement of the Good Friday liturgy to the afternoon (The change in time was accompanied in a change of nature for this liturgy, altering it from a true Mass of the Presanctified to a Communion Service Outside Mass focused on the Passion and Death of our Lord).

This change in time did restore these liturgies to their older usage as practiced during the first millennium. However, arguing this as a justification for the changed times dangerously approaches the error of liturgical antiquarianism. On the other hand, the time change for the Holy Week Triduum Liturgies did make them more convenient for working persons and state-school students to attend.

Question:  May the times at which the Pre-1955 Triduum Liturgies are celebrated be modified? May a traditional priest of his owe accord choose to celebrate the Pre-1955 Triduum Liturgies at the times mandated for the Reformed Holy Week Triduum of Pius XII?

I have my own opinions on this. I am interested to read the opinions of others. Of course priests of the neo-SSPX and the CMRI do not use the pre-1955 Holy Week, and priests of the FSSP and ICKSP who do use the pre-1955 Holy Week are bound to follow the directives of their superiors (and ultimately the current Roman Dicastery for Divine Worship). So this question is really focused on independent priests -- Resistance, R&R, sedevacantist, and sedeprivationist.

This change in time did restore these liturgies to their older usage as practiced during the first millennium. However, arguing this as a justification for the changed dangerously approaches the error of liturgical antiquarianism. On the other hand, the time change for the Holy Week Triduum Liturgies did make them more convenient for working persons and state-school students to attend.

I believe that arguing SOLELY on the basis of "older is better" is where antiquarianism comes in, whereas it would be perfectly reasonable to argue that there were some incongruities between the Rites (their language and symbolism) and the shifted time of day, to say nothing of the fact that celebrating Easter Mass on Holy Saturday morning creates a lot of confusions when Christ rose on the Third Day.  That isn't an argument resting solely in antiquity.

If you look at how Maxima Redemptionis was worded ...

In the beginning these rites were celebrated on the same days of the week and at the same hours of the day at which the sacred mysteries took place. Thus the institution of the Most Holy Eucharist was recalled on Thursday, in the evening, at the solemn Mass of the Lord’s Supper. On Friday a special liturgical service of the Lord’s Passion and Death was celebrated in the afternoon hours. Finally, on the evening of Holy Saturday the solemn vigil was begun, to be concluded the following morning in the joy of the Resurrection.

But in the middle ages, for various concomitant reasons, the time for observing the liturgy of these days began to be anticipated to such a degree that – toward the end of the middle ages – all these liturgical solemnities were pushed back to the morning hours; certainly with detriment to the liturgy’s meaning and with confusion between the Gospel accounts and the liturgical representations referring to them. The solemn liturgy of the Easter Vigil especially, having been torn from its own place in the night hours, lost its innate clarity and the sense of its words and symbols. Furthermore, the day of Holy Saturday, invaded by a premature Easter joy, lost its proper sorrowful character as the commemoration of the Lord’s burial.

One might simply argue that those "concomitant reasons" that led to the change THEN no longer come into play NOW, and that we have our on reasons to move the times back.

Were the times specifically spelled out in the pre-1955 Missal, or was it just a matter of long-standing custom?

All the conventual Masses of Lent and Passiontide, except the Sundays which were not fasting days, were celebrated after None and before Vespers.  In many of the rites of the religious orders and local rites, Vespers was often interpolated closely into the Mass rite those days.  In the Roman rite that was preserved only on Holy Saturday.

Also the celebrant had to be fasting from Midnight so the norm was the celebration of Mass between dawn and Midday (with an hour variance each way), c.f. R.G. XV of the Missal.