Final answer:
Medieval calendars denoted holidays or saint's days using the color red, which was symbolic of the sacred in the liturgical context.
Step-by-step explanation:
In medieval calendars, holidays or saint's days were typically denoted using the color red. This practice was part of a broader tradition of using specific colors symbolically within Christian and other religious contexts. Red, associated with the sacred and the divine, helped to emphasize the significance of these special days. This use of color played a crucial role in the organization and understanding of ecclesiastical calendars, guiding the faithful through the liturgical year and marking out periods of festivity and veneration.