Decided to put together a changelog of every Splatoon update for funsies. Hopefully I didn't miss anything. All dates are North American, so add a day for Europe and Japan.
Patches, new game modes and Splat Fests are highlighted, 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 and 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
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
July 21: Added: Neo Splash-o-matic, E-Liter 3K Scope
July 24: Added: Camp Triggerfish