Kshaya Tithi
A tithi that never touches a sunrise, and is skipped.
Because a tithi can be shorter than a day, one can begin after sunrise and end before the next — so no civil day 'carries' it. It is then a kshaya (lost) tithi. The reverse also happens: a tithi spanning two sunrises is repeated (vriddhi). Both are ordinary, and both are why a month can seem to have a missing or duplicated date.