3rd day would be tertio die or 3io die. I don't know what dnu is, and I'm not convinced that is a 3. Are you sure this is Latin? What country is it from? Any chance of seeing other entries on the same page or a bigger sample of this handwriting?
That image has a few similar examples. Entries 3 & 5 are clearly Den. I admit the other one looks like Dnu. But these entries seem to be a mixture of Latin, Czech and German (not that I know anything about Czech or German).