(Updated Jun 24 2018)
Expanding on the seasonal activities from the Before You Ask document, here's the main event of each season, popular enough that websites dedicate a special section for them.
on Sakura(Spring Cherry Blossoms)
- JNTO Sakura
- Japan Guide Sakura
- Yokoso Sakura
- Tohoku Sakura Dream Road by Rakuten
- (Japanese) Rurubu Spring Sakura
- (Japanese) Yahoo Sakura
- (Japanese) Tenki.jp Sakura
also during Spring Season
Tide Pool Collecting, Clam Digs
on Hanabi(Summer Fireworks)
- Japan Guide Summer Fireworks
- Japan Attractions Summer Festival & Fireworks
- Japan Talk 20 Best Hanabi Events in Japan
- Go Tokyo Fireworks Festivals
- (Japanese) Rurubu Summer Fireworks
- (Japanese) Walker Plus Summer Fireworks
- (Japanese) Yahoo Summer Fireworks
- (Japanese) Jorudan Summer Fireworks
also during Summer Season
Fireflies Watching
- TokyoCheapo Summer’s Here – Firefly Nights Around Tokyo
- The Expat's Guide to Japan - Hunting Fireflies
- Quest for Japan - Firefly Watching
- (Japanese) Rurubu Summer Fireflies
- (Japanese) 2016年 今年は見たい！ホタル（蛍）観賞のポイントあれこれ
- (Japanese) Walker Plus Summer Fireflies
- (Japanese) Iko-yo Summer Fireflies
- (Japanese) Weathernews Summer Fireflies
Fruit Picking
- Japan Guide Fruit Picking
- JNTO Summer Fruit Picking
- (Japanese) Rurubu Summer Fruit Picking
- (Japanese) Kudamono Navi Fruit Picking
- (Japanese) Iki-yo Fruit Picking
- (Japanese) Rankingshare Summer Fruit Picking in Nagano
Pool & Beach
on Koyo(Autumn Leaves)
- Japan Guide Autumn Colours
- JR East Autumn Colours
- Kyuhoshi Autumn Foliage Forecast
- (Japanese) Rurubu Autumn Koyo
- (Japanese) WeatherNews Koyo
- (Japanese) Tenki.jp Koyo
also during Autumn Season
Autumn Harvest
- Navitime Autumn Fruit Picking
- (Japanese) Rurubu Autumn Mikaku
- (Japanese) Iko-yo Autumn Mikaku
on Light Up(Winter Illumination)
- Japan Guide Winter Illumination
- CNN Technicolor triumph: Japan's dazzling light decorations
- Nabana no Sato Winter Illumination
- Winter Lights in Japan
- (Japanese) Rurubu Winter Illumination
- (Japanese) Walker Plus Winter Illumination
- (Japanese) Osaka Night Xmas Illumination (suggested by Allan Brewster)
also during Winter Season
Ski Resorts
- Snow Japan
- Powder Hounds Japan
- J2Ski
- (Japanese) Snownet
- (Japanese) GNAVI Surf & Snow
- (Japanese) Tenki.jp Ski
- (Japanese) Jalan Ski
Christmas Festivities
- Where can I visit Santa in Tokyo? (written by Lynda Hogan)
- (Japanese) Tokyo Christmas Events
- (Japanese) Walker Plus Christmas Event Guide
Shogatsu (New Years Countdown, First Temple Wish, First Sunrise, ...)
- New Year In Japan
- (Japanese) Rurubu Winter Shogatsu
- (Japanese) Jorudan Shogatsu
- (Japanese) Walker Plus Shogatsu
Fukubukuro (New Year Lucky Bags)
- Matcha - The Fun Of "Lucky Bags"
- (Japanese) Naver - Popular Fukubukuro
- (Japanese) Retrip - Fukubukuro Top 10
Ume (Plum Blossoms, usually a month or two before Cherries begin to blossom)
- Japan Guide Plum Blossom
- CNN 10 Best Plum Blossom Tokyo
- (Japanese) Rurubu Winter Plum Blossom
- (Japanese) Hanazakura Winter Plum Blossom
- (Japanese) Navitime Winter Plum Blossom
Ichigo (Strawberries Picking, Japan's season is usually between mid December to early May)