That would be a good explanation except for the fact that all services of Holy Week are transferred forward to the evening. We serve the Matins for Holy Monday on Sunday evening and so forth. The Presanctified Divine Liturgies which we celebrate on Wednesday and in the Russian tradition on Friday evenings during Great Lent are celebrated in the mornings of the first three days of Holy Week. I serve the Divine Liturgy of Holy Thursday at 7 a.m. on Thursday, although from the text we can tell that it was originally celebrated in the evening.
Christ is risen!
I'm aware that the morning services in Holy Week are also anticipated the evening before, but I would suggest that if the Vesperal services are all moved to the morning to accommodate Communion without an overbearing fast (which is my supposition), there really is no where else to put the morning services.
My understanding of the rubrics (based on a recollection of what is in Met Kallistos' translation of the Triodion) is that, with the exception of the Matins of Good Friday, most of the morning services of Holy Week are actually supposed to be served early in the morning of the day itself (e.g., Matins of Holy Tuesday should start around 1am or 2am on Tuesday, not at 7pm on Monday evening). I suppose these are supposed to be vigils throughout the night lasting until dawn. The fact that the Matins of Friday is much lengthier due to the Gospel readings and extra hymnography probably justifies its earlier start time in the rubrics. There is a pastoral advantage in parishes to having these services anticipated in the evening rather than keeping them at their intended time, of course, but the rubrics don't appear to intend that the morning services will be anticipated any earlier than "very
early in the morning" except in the case of Friday.
Regarding Vesperal services in Holy Week, I find it interesting that every one of them is served in the morning except on the one day when there is no Communion at all and the fast is supposed to last all day: Good Friday. Every other day, there is some form of Communion attached to Vespers (be it the full Liturgy on Thursday and Saturday or the Presanctified Liturgy on Monday through Wednesday), and it is usually anticipated in the morning, but the earliest I have ever seen the Vespers of Friday served is noon, but more often it is somewhere between 2pm and 4pm. So on the one day where Communion is not a concern, the services are served more or less at their intended times. Even during the weeks of Lent, the Presanctified Liturgy, at least in my area, is served in the morning at least as much, if not more, than it is served in the evening. So I think Communion is really the deciding factor in the timing of the Vesperal services, and the rest of the daily cycle probably revolved around that. Just a hunch.
Do the Oriental Orthodox also move their Holy Week services forward so that a morning service is served the evening before as the Eastern Orthodox do?
As far as I can tell, no. Speaking for Holy Week in the Syriac tradition as practiced in Indian communities, morning services are served in the morning, and evening services in the evening. We have no Presanctified Liturgy on Monday through Wednesday, so that's not really an issue on those days. The Liturgy on Thursday is not a Vesperal Liturgy according to the rubrics: it is an early morning Liturgy, traditionally beginning with the Midnight Office around 2am, Matins and Third Hour around 3.30-4am, and the Liturgy immediately following, ending about daybreak. The services of Friday literally last all day (the average parish's Friday services run from 9am-3pm), so we don't worry about the intended time because we're already there.
The Liturgy of Saturday is a fasting Liturgy: it is served after the Ninth Hour of Saturday, but before Vespers (of Pascha). It is not a Vigil for Pascha as it is in the Byzantine rite, but is the Liturgy of the day. Its "in-between" character can be seen not only in its traditional time, but also in its readings: the Epistles are baptismal and Paschal in nature, but the Gospel is St Matthew's account of the burial of Christ and the meeting of the Jews with Pilate on Saturday to arrange to have the tomb guarded. The rubrics and music of the Liturgy are also penitential and not joyous, but it is supposed to conclude with the rite of Forgiveness (as at the beginning of Lent) as a way of ushering in the Paschal celebration beginning later in the evening.
With some variation, this seems to be true of all the OO except possibly the Armenians, whose schedule looks more like the contemporary Byzantine practice. But these two rites have other similarities as well, and I suspect the practice of the Church of the Resurrection in Jerusalem has been influential in this regard, at least in part.