Passiontide is the week prior to Holy Week. It begins with Passion Sunday and ends when Palm Sunday is ushered in. Passion Sunday is the Fifth week of Lent. It marks a movement in the Liturgical Kalendar when we go from highly focusing on our sins, to highly focusing on the necessity of Christ Crucified for the redemption of our sins and those of the whole world.
Customs at this time include the veiling of images, if not done already on Ash Wednesday (an old Sarum Rite custom). If the veils were already out all season, the crucifix might be veiled at this time. This comes from the old lectionary where John 8:46-59 is read, recounting Christ "hid" from those around the Temple who would stone him for declaring his divinity, "Before Abraham was, I AM." Now he hides from us. What does that say about us? Would we join the Pharisees who Jesus mocks? "And say, If we had been in the days of our fathers, we would not have been partakers with them in the blood of the prophets" (Matthew 23:30).
There is also the removal of the "Glory Be to the Father and to the The Son and to the Holy Ghost, as it was in the beginning, is now, and ever shall be, world without end. Amen" from the beginning of the Morning, Evening, and other prayers. (I.E. The Glory Be disappears from the Invitatory of the Offices.) After Wednesday of Holy Week the Glory Be disappears from everything, including after Canticles and Psalms in Morning & Evening Prayer. This mounts to a building pressure so that by the time "God dies" on Good Friday, there is no Alleluia nor Glory be left to be said in the wake of sorrow. There is no Glory Be as the darkness approaches. And we, like the Apostles, have fled from him, retreated, and performed treason by our sins. The Glory Be does not return until The Easter Vigil, when Christ arises.
Each day of Passiontide is a distinct day with its own propers (like every day in Lent). They are called Monday, Tuesday, Wednesday, and Thursday of Passion Week. Friday is known as "St. Mary in Passiontide." In some other countries, it has other names as well: Our Lady of Sorrows, Friday of Sorrows, Our Lady in Passiontide, &c. On this day the Stabat Mater, or "Sorrowful Mother" is the Sequence for the Gospel (it's that long chant before the Gospel, like the ones you hear at Christmas, Easter, & Whitsunday). You may recognize this tune from the Stations of the Cross, "At the cross her station keeping // sat the mournful mother weeping..." This day commemorates the sorrow of Mary at watching her son die upon the cross.
The Friday of Passiontide, St. Mary in Passiontide commemorates the fulfillment of the prophecy told to St. Mary by St. Simeon. Mary brought the Christ child to the Temple to be purified. Simeon, upon seeing the infant, declares, "Lord now lettest thou thy servant depart in peace," which essentially declares that he awaited the coming of the Messiah, The Christ, the Anointed One or King of Israel, and having seen him, he can now die in peace. We sing Simeon's Song (Nunc Dimittis) at Evening Prayer every night and/or at Compline from The Daily Office. After this, Simeon tells Mary that "a sword shall pierce your own heart as well." This sword pierced her heart in the form of grief at seeing her crucified son and holding his dead body after it descended from the cross. This day (today), we have compassion on our mother, for her love was to give up her son for the redemption of the world.
St. Mary in Passiontide is thus a mirror image and foreshadowing of her son's Passion the following Friday.
Passiontide ends with the Council of the Sanhedrin meeting to discuss how they will capture Jesus. Many have come to Mary looking for him, and have come to believe in him. Now the Passover feast approaches, foreshadowing Jesus' Passover on Maundy Thursday, and the Sanhedrin's kangaroo court on the night of Maundy Thursday, which is the beginning of Good Friday.