About Auspicious Days and the Lunar Calendar
In the Tibetan lunar calendar sometimes certain dates do not appear or certain dates occur twice. According to the Mindrolling tradition, if an auspicious lunar day is missing, it is observed on the day before. For example, if the 10th lunar day is missing that month, you would observe Tse Chu on the 9th lunar day. If an auspicious lunar day is doubled, the practice is observed on the second day. For example, if there are two 10th days in a lunar month, then you would observe Tse Chu on the second of the two.
Mindrolling Monastery Calendar