This reliquary contains the Crown of Thorns of our Lord as recovered by King St. Louis IX.
The first feast in honour of the Crown of Thorns (Festum susceptionis coronae Domini) was instituted at Paris in 1239, when St. Louis brought thither the relic of the Crown of Thorns, which was deposited later in the Royal Chapel, erected in 1241-8 to guard this and other relics of the Passion.
The feast, observed on 11 August, though at first special to the Royal Chapel, was gradually observed throughout the north of France. In the following century another festival of the Holy Crown on 4 May was instituted and was celebrated along with the feast of the Invention of the Cross in parts of Spain, Germany, and Scandinavia. It is still kept in not a few Spanish dioceses and is observed by the Dominicans on 24 April.
A special feast on the Monday after Passion Sunday was granted to the Diocese of Freising in Bavaria by Clement X (1676) and Innocent XI (1689) in honour of the Crown of Christ. It was celebrated at Venice in 1766 on the second Friday of March.
In 1831 it was adopted at Rome as a double major and is observed on the Friday following Ash Wednesday. As it is not kept throughout the universal Church, the Mass and Office are placed in the appendices to the Breviary and the Missal. The hymns of the Office, which is taken from the seventeenth-century Gallican Breviary of Paris, were composed by Habert. The "Analecta hymnica" of Dreves and Blume contains a large number of rhythmical offices, hymns, and sequences for this feast.
I recommend the meditation on today's historic feast at The Guild of Blessed Titus from a 2013 post. Also, the meditation shared on the Irish Dominican blog from 2010's Feast is also very worth reading this day as a source of meditation.
Grant, we beg, almighty God: that we, who in memory of the passion of Our Lord Jesus Christ do revere His thorny Crown on earth, by Him may deserve "to be crowned with glory and honour" (cf. Ps 8:6) in heaven: Who with Thee liveth and reigneth...