I used the spelling from the thread header but I did check out dictionary.com and that spelling of it (Decenallia) is not there. They did give me this tho:

Decennalia (or decennia, from Latin decennium - "tenth anniversary") was an Ancient Roman festival celebrated with games every ten years by the Roman emperors. This festival owed its origin to the fact that Augustus refused the supreme power when offered to him for his life. Instead, he would only consent to accepting power for ten years, and, when these ten expired, for another ten years, and so on to the end of his life. The memory was preserved to the last ages of the empire by Decennalia, which was solemnised by subsequent emperors every tenth year of their reign, although they had received the imperium for life, and not for the limited period of ten years.

So in light of that, I suggest that we use the dictionary.com spelling. I'm sure any plaque can be changed to accomodate the right spelling.