This perpetual calendar numbers days in sequence across rows of trecenas of 13 days each. Each day is named first for the day number at the column head followed by the day name in the row. For example, the zeroth day is 13 Flower, followed by the first day 1 Light, 2 Wind, then 3 Temple, etc. The numbers cycle continuously from 1 to 13 and the days cycle through 20 pictures. Find the last day-name (Flower) on the second row and notice that it is followed by the first one again (Light). It takes 20 trecenas for the entire sequence to begin again on 1 Light, for a total of 20 x 13 = 260 days in the Sacred Round. Note also that each trecena of 13 days is named for its zeroth day, which names are in bold face. For example, the first trecena is named Flower, and the second trecena is Reed. There are 20 trecenas in the Sacred Round.