They were political documents, lavish self-promotional billboards aimed at asserting the wealth and stature of the host.
... Printing became available in the 15th century and within 100 years, the royal and aristocratic courts behind Europe's most illustrious festivals printed elaborate, illustrated volumes to record these celebrations. "This was not just a souvenir, but a propaganda item," said Babcock. "The ruler of the other states needed to know that the duke of Tuscany's daughter married a Hapsburg prince to show all the other rulers that they were important, they were wealthy and they do great things for their people."
... The books themselves vary in their level of documentation. Many, like a 17th century festival for Santa Rosalia, patron saint of Palermo, feature folded-out engravings of the ceremonies along with painstaking citations of who was there. The most breathtaking example here is a 19th century volume commemorating the Roman feast of Corpus Christi. It includes a single color illustration that, unfolded, is more than 38 feet long. Each of the thousands of participants of the procession, including Pope Gregory XVI, is identified.
The exhibit continues through January 9th. (Thanks to Rare Book News for this tip!)