Kshaya Tithi is when a tithi or lunar day completely misses to be marked in a calendar or is expunged.
Tithi is a basic unit in the Hindu calendar and panchang. Tithi is the lunar day. A lunar month consists of, normally, twenty eight days or twenty nine and half solar days.
A tithi may begin at any time of the solar day but, for the practical purpose of recording dates, that tithi current at the sunrise is supposed in the assigned fortnight.
However, it that tithi begins just after the sunrise and ends before the sunrise of the next day, it is expunged. That is called Kshaya Tithi.
Kshaya Tithi is considered inauspicious and it is not chosen for any kind of auspicious activities.