Going backwards in time from most recently instated, today is:

Children's Day (こどもの日): Nationwide public holiday and the end of Golden Week. We are also supposed to appreciate our mothers on this day. Thanks, mum!

The Boy's Festival (端午の節句): Apparently, the word 端午 (tango) -- literally "Beginning Horse", perhaps -- originally referred the fifth day of any given lunar month (the fifth day being the first Horse day in the month), and the の節句 (no sekku, roughly "seasonal festival") was later added to distinguish between all these other tangos and the one that occurs on the fifth month of each year, which is specifically the Boy's Festival. Clear?

So the carp streamers and things are not really for the postwar Children's Day itself, but rather for the older festival, Boy's Day. This is why you see them referred to in old sayings like the one Wikipedia helpfully mentions:

江戸っ子は五月の鯉の吹き流し、口先ばかりで腸は無し
Edokko wa gogatsu no koi no fukinagashi, kuchisaki bakari de harawata wa nashi

Edo folks are May carp streamers, all mouth and no guts

Zing! That's a 5-7-5-8-7 syllable structure, by the way. I'm not sure if it's a senryuu + explanatory comment or a satirical tanka. (And do I dare hope that the rhyme is intentional?)

And finally, this year, today is also the beginning of the calendar section after Kokuu -- namely, 立夏 (rikka), which means "Summer Begins". Yes! It is now officially summer in Japan. Cicadae who do not start going min min, jiit jiit or at the very least kanakanakana as of this evening are liable to heavy penalties.

The three subseasons of Rikka would be:

1. 蛙始鳴, kaeru hajimete naku: Frogs Begin to Call. (Or, in China, 螻蝈鳴, which Wikipedia says means the same thing except it specifies tree frogs.)
2. 蚯蚓出, kyuuin izuru: Worms Come Out.
3. 竹笋生, chikukan shouzu: Bamboo Shoots Emerge. (Or, in China, 王瓜生, which means "王瓜 Emerge/Begin to Grow/etc." I'm not sure but I think those are called "snake gourds" in English.)