On Great and Holy Saturday the Altar is brought down into the center of the church and the liturgy is served there.
What is this bizarre practice?
The altar is stripped Great and Holy Friday. The DL is celebrated on the Epitaphion, as it is celebrated in the Holy Sepulchre/Edicule in the Church of the Resurrection. The Epitaphion and antimensis are returned to the altar at the Paschal Vigil during the last Ode.
Oh yes, that's right... it's not the altar it's the tomb with Christ (Epitaphion) on top of it. Sorry for the confusion.
I must admit to still being confused by this. I have never seen the Holy Table stripped or the Holy Saturday Liturgy served on the plashchanitsa in the midst of the nave but only on the Holy Table.
This leads me to another question I've wondered about. Why do we celebrate the liturgy and receive communion on Great and Holy Saturday? I can certainly understand having a service but I dont quite understand the white vestments and celebrating the Eucharist before the Paschal services in the evening.
The Paschal Vigil that is currently served from Holy Saturday night to the early morning of Pascha is a later development. At a talk given by Metropiolitan Kallistos of Diokleia, he suggested this was possibly around the 7th-8th centuries.
What is now the Vesperal Divine Liturgy on Holy Saturday was originally the night vigil, and anybody who comes from a western Christian background will recognise this because it has the same roots as the Western Paschal Vigil. As an Anglican, this was the first Orthodox service I ever attended and some of the similarities struck me immediately. Both east and west have numerous readings from the Old Testament, many of which are the same. The Gospel readings are both from Matthew 28. In both east and west, the service starts with the clergy in dark vestments and dark cloths on the Holy Table and icon-stands, and these are changed to white/silver vestments part-way through. Baptisms take place in the western vigil and we have the remnant of that in the Byzantine vigil with As many as have been baptised into Christ
replacing the Trisagion, and both culminate with the celebration of the Eucharist. It is clear that this is the original paschal service which must have taken its form and content early enough to be common to both east and west, (although it then developed differently within the context of each rite).
Yet for some reason, in the Byzantine Rite, when the new vigil was introduced, the old vigil was not replaced but was simply moved to earlier in the day and combined with Vespers. Now it is served on Holy Saturday afternoon. (I understand that the ustav prescribes it for 4 p.m. although at my parish it usually starts between 2.30 and 3 p.m. I've heard that some parishes even serve it in the morning!) So now, what we essentially have is two paschal vigils, although we are told that we must not greet each other with the news of the Resurrection until the second vigil. Really, while it's all very lovely, it's a bit of a liturgical mess.
However, what we find at our parish is that the people who would struggle to come to the night vigil, (elderly people, people with very young children and a long distance to travel, people who rely on carers, and so forth), come to the afternoon vesperal vigil, and afterwards they bring their baskets to be blessed, they greet each other with the paschal greeting, and they break their eggs together - all festivities that would usually not be expected until the night vigil. Then they go home, satisfied that, while they might not have endured the night, they have come together and given honour to the Lord's Resurrection. So while having two vigils may be nonsense liturgically, it is actually very useful pastorally.
I don't know whether that helps.