Super duper necropost but I was re-reading through this.
Adding the other references that the earlier post didn't cover:
Meme 10/10 = Eye Care Day = based on the idea that 10 when turned 90 degrees looks like an eye with an eyebrow over it
Chiyo 10/1 = Glasses Day (because 10/01 could be written as ー00ー which looks like glasses)
Yamame 8/11 = Mountain Day (was originally proposed to be August 12th, but that was the day of the terrible Japan Airlines 123 air crash so they moved it one day forward)
Momiji 4/8 = Shiatsu Day = as in a massage, from shi (4) atsu (8)
Kishika 10/10 = Baby's Day = under the old lunar calendar where a month was 28 days, "10 months and 10 days" was the typical duration of pregnancy
Ahko 2/5 = Smile Day since 2/5 is "niko" and "nikoniko" is the onomatopoeia for smiling
Uto 5/16 = Traveler's Day = said to be the day that the poet Matsuo Bashou set out on the journey during which he wrote the
Oku no Hosomichi
Eira 8/3 = Capoeirista Day
For other birthdays:
Tama 2/22 = Cat's Day = from nyan nyan, sound of meowing
Himeka 10/31 = 天 (ten = 10) 才 (sai = 31) meaning "prodigy"
Matsuri 8/15 = Obon Night
Shiina 4/17 = Shi (4) + ina (17)
Meru 11/30 = Storybook Day = the day that Teiji Seta published his collection of children's stories, 絵本論
Saki 4/1 = the first birthday in a given fiscal year (4/1 through 3/31)
And of course,
Rentarou 5/1 = ko (5) i (1) = "love"