I often see dates expressed as, for example, tertio idus maias where idus is the feminine accusative plural and maias is the adjective agreeing with idus relating to the month of May. So, the question is how to interpret tertio which can either be an adverb mean "thirdly", or potentially as the ablative form of the adjective tertius meaning third which is modifying an implied die. In other words, tertio (die) idus maias ("on/regarding the third day before/towards the ides of May"). An example of a full sentence is:
Tertio idus Maias Fidis mane oritur, significat tempestatem. ("Third ides of May, the Lyre rises in the morning, it signifies a storm.")
So, the question is: is tertio the adverb or the ablative of the adjective tertius?