There are a few synonyms for the word "thirteenth" which can be used in place of it, but still, convey the same meaning. One such synonym is "13th", which is a shortened version of "thirteenth". Another synonym that can be used is "tredecim", which is a Latin term for 13th. A third synonym for "thirteenth" is "13th day", which can be used when referring to a specific date. "1-2-1" and "1-1-11" can also be used to denote the thirteenth day of the month. These synonyms could be used in writing or conversation to add variation and interest to a sentence.