Many have the custom to make a seder for Tu Bishvat. When Tu Bishvat is on Shabbos as it is this year, when is the optimal time to make the seder? If as part one of the three Shabbos meals, which one? If as a separate seder, in between which meals?
For the general question of whether the Tu B'Shvat Seder should take place by night or day, see here. Of note is the Chemdas Yamim, the earliest source for the Seder (by incorporating the Pri Etz Hadar), who uses the term בעצם היום הזה which generally implies daytime.
However, the Sefer Mo'ed L'Kol Chai (by R. Chaim Palagi), in section 30:7, says: ליל ט״ו בשבט ר״ה לאילנות ונהגו רוב תפוצות ישראל לסדר בשלחן מכל פירות. Further, he writes in 30:8: "There are those who teach their young students all 15 Songs [of ascension, the Shir HaMaalos] in order that they'll be able to teach it at their father's table on the night of Tu B'Shvat". Likewise, the Even Yisrael brings a number of sources that it should ideally be held at night (and some for the day). Last but not least, in the Kitvei Ramchal it is referred to as the סְעוּדַת לֵיל ט״וֹ בִּשְׁבָט.
In regards to your question, R. David Sassoon (d. 1864) writes of the Jews in Baghdad in his מסע בבל (in מהדורת מ. בניהו, ירושלים תשט"ו, עמ' רכ"ו) that on Shabbat the Seder is held during the day since the Jews aren't accustomed to leave their homes on Friday night. To quote:
ליל ט"ו בשבט. מביאים כל מיני פירות, טריים ויבשים, המצויים בעיר, ומניחים על השולחן ועורכים מסיבה וקוראים על כל מין ומין מקרא, משנה ובו מספר פרי עץ הדר. וגם חל בשבת עושים את סדר ט"ו בשבט ביום, מפני שבליל שבת אין רגילים לילך חוץ לביתם.
However, it should be noted that he is reporting that custom based on the local custom in Baghdad for Jews not to leave their homes at night. One could certainly argue that in other situations, the default proper time - evening - would be optimal.