Here's an updated changelog of every Splatoon update for funsies. All dates and Splatfests are North American.
Patches and new game modes are highlighted, Splatfests are italicized, new stages are bold, new weapons are plaintext:
May 29: Splatoon released (29 weapons plus three Amiibo-unlocked weapons, five maps, Battle Dojo, Octo Valley, Regular Battles, Turf War)
June 1: Added: Ranked Battles, Splat Zones, Port Mackerel, N-ZAP '85
June 2: Patch Version 1.2 (fixed a glitch with Spyke not taking money and stabilized Booyah Base framerate)
June 5: Added: Inkbrush
June 10: Added: Kelp Dome
June 12: Added: Splash-o-matic
June 16: Added: L-3 Nozzlenose, Custom E-Liter 3K
June 19: Added: Bluefin Depot
June 23: Added: Luna Blaster
June 26: Added: Custom Dual Squelcher, Carbon Roller
June 30: Patch Version 1.3 (with balance patch: Kraken, Ninja Squid and Stealth Jump nerfed)
July 1: Added: Tower Control
July 4: First Splatfest (cats vs. dogs)
July 7: Added: Sploosh-o-matic, .96 Gal Deco
July 10: Added: Moray Towers
July 17: Added: N-ZAP '89, Octobrush
July 18: Second Splatfest (rollercoasters vs. waterslides)
July 21: Added: Neo Splash-o-matic, E-Liter 3K Scope
July 24: Added: Camp Triggerfish
July 31: Added: Range Blaster, Inkbrush Nouveau
Aug. 5: Patch Version 2.0 (added: Squad Battles, Private Battles, Slosher, Heavy Splatling, three new background songs during matches, new gear in shops; increased level cap from 20 to 50 and ranked cap from A+ to S+; reworked Urchin Underpass; nerfed Inkzooka and Echolocator and buffed Killer Wail)
Aug. 8: Third Splatfest (marshmallows vs. hot dogs)
Aug. 14: Added: Rainmaker
Aug. 20: Added: Flounder Heights
Aug. 29: Fourth Splatfest (Autobots vs. Decepticons)