Using the traditional dates of the Roman calendar, December 31 and December 30 would be pridie Kalendas Ianuarias and ante diem tertium Kalendas Ianuarias. The day is expressed in relation to the first day of the next month. This happens for all days after Idus. For more about writing dates, see this recent question.
If I want to include the year in my date, which year is it? The one of the Kalendae Ianuariae or the one of the day itself? If I want to describe my plans for the end of this year, I seem to have two options:
On December 31, 2016 I will sleep.
Pridie Ianuarias anno 2016/2017 dormiam.
My guess would be 2016. But if I used anni instead of anno instead — which I find unusual but grammatical — then I would choose 2017.
Which year should I choose and why? Have both options been in use? Have grammarians or other scholars commented on this issue?