Spanish Southern Textualis Praescissa.

The entire recto is taken up with the rubric for his feast and an accompanying illustration, in which a crowned woman sits at a keyboard with three children, apparently singing from partbooks. Decorative motifs fill in the page, including a dog/dragon and a trumpet-blowing sun surrounded by colourful foliage.
On the verso, the M (for "Magno") has a more rustic quality, in blue and red with daisy-like flowers in the middle. The initial I which follows has typical Spanish penwork in black, but enhanced with touches of blue, red, and light blue.

Likely Spanish or Spanish Americas; possibly from the Hieronymite order, given the prominence apparently given to Jerome, or a church dedicated to him.