I am remiss in not having yet read the 2001 document from the Congregation for Divine Worship and the Discipline of the Sacraments, Directory on Popular Piety and the Liturgy. But, since I've always thought a facility for quotation covers the absence of original thought, let me quote from the document's section on Holy Thursday:
Visiting the Altar of Repose
141. Popular piety is particularly sensitive to the adoration of the Most Blessed Sacrament in the wake of the Mass of the Lord's supper. Because of a long historical process, whose origins are not entirely clear, the place of repose has traditionally been referred to as a "a holy sepulchre". The faithful go there to venerate Jesus who was placed in a tomb following the crucifixion and in which he remained for some forty hours.
It is necessary to instruct the faithful on the meaning of the reposition: it is an austere solemn conservation of the Body of Christ for the community of the faithful which takes part in the liturgy of Good Friday and for the viaticum of the infirmed. It is an invitation to silent and prolonged adoration of the wondrous sacrament instituted by Jesus on this day.
In reference to the altar of repose, therefore, the term "sepulchre" should be avoided, and its decoration should not have any suggestion of a tomb. The tabernacle on this altar should not be in the form of a tomb or funerary urn. The Blessed Sacrament should be conserved in a closed tabernacle and should not be exposed in a monstrance.
After midnight on Holy Thursday, the adoration should conclude without solemnity, since the day of the Lord's Passion has already begun.
What I give in the post below, then, in terms of when which day begins, the CDWDS takes back here. So it goes.