Among the major works of late medieval figurative art in its possession, the ancient Collegiate Church of Sts. Peter and Orso in Aosta has a work of great historic value: the Missale Magnum Festivum Georgii Challandi, which was commissioned in the late 15th century by a descendant of the most powerful and illustrious feudal family in the Aosta Valley, Giorgio di Challant, the prior of the Aosta Collegiate Church from 1468 to 1509 and governor of the Duchy of Aosta.
This is an illuminated manuscript of a missal that is quite large (51x34 cm) and comprises 176 parchment folios (hence 352 pages), the first eight and last seventeen of which are not numbered. The size of the text proper is 33x21 cm and each page contains 28 lines of text in two columns.
The illuminations are of high artistic quality, as can be seen in the large plate of the Crucifixion that precedes the Canon, and those representing the Conception of the Blessed Virgin Mary, the Nativity, the feasts of St. Orso, St. Peter and Paul, St. John the Baptist, Easter, the Pentecost, Corpus Domini, the Assumption of the Virgin Mary, the Birth of the Virgin Mary and All Saints' Day. In these works the decorative and figurative elements, which often draw inspiration from nature, trascend merely stylistic decoration to become a necessary and expressive part of the entire iconographic layout and the text.
Besides the many scenes represented – true masterpieces of a particular art form that differs considerably from the Italian schools of the time – the decorations in the margins with their warm, vivid colours and the large initial decorative letters, many of which are rendered in gold leaf, reveal the artist's uncommon ability in varying the arrangement of the decoration without lapsing into repetition.