Tibetan calendar
A Tibetan calendar month normally starts with the week day or natural day ("gza' " or "nyin zhag") in which the first tithi ("tshes zhag") ends. A Tibetan calendar month normally ends with the week day or natural day ("gza'" or "nyin zhag") in which the 30th tithi ("tshes zhag") ends. In consequence, a Tibetan calendar month ("zla ba") comprises 29 or 30 natural days. In the sequence of natural days or week days, there are no omitted days or days that occur twice. But since these days are also named by the term "tshes" together with a cardinal number, it happens that certain numbers or dates (the corresponding tithi) do not occur at all ("chad") or appear twice ("lhag"). The tithi are counted from 1 to 30 and it can happen that a Monday with the lunar day number 1 ("tshes gcig") is followed by a Tuesday with the moon day number 3 ("tshes gsum"). On the other hand, a Monday with the lunar day number 1 ("tshes gcig") may be followed by a Tuesday with the lunar day number 1 ("tshes gcig"). In other words, it happens quite often that certain dates do not appear in the Tibetan almanac and certain dates occur twice. But there are no natural days or week days that occur twice or which are omitted.