What you speak of is known as the Interrupted Liturgy or the Unfinished Liturgy.
According to tradition, the Divine Liturgy being served at 'Agia Sophia on Tuesday, 29 May 1453, was concelebrated by two presbyters, Catholic and Orthodox. (HB Isidore, Patriarch of Kiev and all Russia, had declared a disputed union of East & West some 5 or 6 months prior.)
When the invaders approached the Holy Table, the southern wall is said to have opened at the touch of an angel and at his direction the presbyters are said to have miraculously passed through the the wall, with the Holy Gifts in hand.
The door that had appeared in the solid wall closed behind the priests and reportedly will not reappear and be reopened until the Interrupted Liturgy can be resumed. It is said that when that day comes, the priests will re-enter through that same doorway by which they departed.
For many years, our Greek Orthodox brethren commemorated this Liturgy each Tuesday. I don't know for certain if that is still the case - perhaps Alice or Father Anthony can speak to that.