The Pri Etz Hadar, as incorporated in the Chemdas Yamim, tells us of the idea of making a Tu B'Shvat Seder. It seems that most people (such as here, here and here) assume the Seder is meant to take place at night. I conjecture that they take the parallel to the Pesach Seder as an indication of the time of day. However, the Chemdas Yamim does not say clearly when the Seder is indeed meant to occur. In fact, in a number of places (such as here in the first paragraph) he uses the term בעצם היום הזה, which I would take to mean that the Seder should be had at noontime or at least during the day, as that phrase usually means.Does anyone know of a clear source text that states when the Seder should take place?
Well there's this Rabbi Adir HaKohen who brings a small siman (from Rosh HaShanah daf 14b) for why the focus is the evening.
In addition, here's a collection of halachos for Tu BiShvat, where the rabbi brings one source for day (Even Yisrael) and two for night (Yafe LaLev and Moed Lekol Chai). Do not know these sources well enough, sorry :)