Deipnon is the night of the dark moon and the following daylight hours. The first step to determining when Deipnon will fall in your area is to look up the time and date of the astronomical new moon (which is actually the dark moon) and sunrise.
If the astronomical new moon is before sunrise, you can count that night as Deipnon and the next night as the Noumenia. If the astronomical new moon is after sunrise, that evening is Deipnon. There is some room for a judgement call on your part, though, when the times are particularly close.
Let’s use February 2019 in Henan, PR China for our example since that’s the next Deipnon and where I am. The astronomical new moon is February 5th at 5:03am. Sunrise is at 7:21am. In the most technical manner of speaking, we could consider those two hours and 18 minutes to be enough to make Deipnon begin at sundown on February 4th and continue until sundown on February 5th, but I generally prefer a bigger margin, or at least for the dark moon to not happen at a time that I think of as morning. This, however, would be your call. I will celebrate Deipnon from sundown on February 5th until sundown on February 6th. Incidentally, the Chinese, Jewish, and Muslim lunar calendars agree with me on this one, but they don’t always. Hellenic polytheists cannot reliably count on one of these other large groups with many functioning calendar apps to do the legwork for us. It’s something that we really need to calculate for ourselves.
I am curious about your take on this judgement call that I’m making. What would be your preference and why?