━ Please keep in mind that these are not official information and some may not be complete.
━ KARASUNO
Hinata (21 June) = Summer Solstice.
Kageyama (22 Dec) = Winter Solstice.
Tsukishima (27 Sep) = Harvest Moon.
Yamaguchi (10 Nov) = Hachikou's birthday.
In Japan, Hachikou was known as Chuuken Hachikou (Faithful Dog Hachikou) in which the kanji for Chuu from Chuuken is the same as the kanji for Tadashi.
Nishinoya (10 Oct) & Azumane (1 Jan) = 10/10 ⇔ 01/01.
Shimizu (6 Jan) = Jeanne d'Arc's birthday.
━ AOBA JŌHSAI
Oikawa (20 July) = the day Alexander the Great was born
Iwaizumi (10 June) = the day Alexander the Great died.
Kindaichi (6 June) = Japan's Rakkyou Day.
━ NEKOMA
Kuroo (17 Nov) = Italy's Black Cat Day. Peak of Leonids.
Kai (8 April) = Buddha's birthday.
Yaku (8 Aug) = World Cat Day.
Kenma (16 Oct) = World Spine Day.
Yamamoto (22 Feb) = Japan's Cat Day.
Fukunaga (29 Sep) = Japan's Manekineko Day.
Inuoka (1 Nov) = Japan's Dog Day.
Shibayama (16 Dec) = the day Shibainu was assigned as Japan's Natural Monument.
Haiba (30 Oct) = the day the October Manifesto was issued by Emperor Nicholas II.
━ DATEKOU
Moniwa (6 Sep) = Kurou no hi (hardship day) because 9/6 can be read as kuro. Pakistan's Defence Day.
Kamasaki (8 Nov) = Japan's Cutlery Day. (as in bladed object/cutting instrument because kama = sickle)
Sasaya (10 Feb) = the day Maeyachi Station's name was changed to Sasaya Station.
Aone (13 Aug) = start of the Berlin Wall's construction.
Futakuchi (10 Nov) = fall of the Berlin Wall.
Obara (15 Dec) = the day Shiroishi Station (the closest station to Obara Onsen) was established.
Sakunami (30 Aug) = the day Sakunami Station was established.