Here's an updated changelog of every Splatoon update, for the curious. 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, 160 pieces of gear in shops plus nine Amiibo-unlocked ones and the Splatfest Tee, five background songs during matches, 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 (same teams can play against each other in Splatfest when there aren't enough players on the other team; Kraken, Ninja Squid and Stealth Jump nerfed; Inkstrike coverage made more consistent)
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, four new background songs during matches and 45 new pieces of 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. 28: Added: Bamboozler 14 Mk I, L-3 Nozzlenose D
Aug. 29: Fourth Splatfest (Autobots vs. Decepticons)
Sept. 1: Patch Version 2.1 (fixed bugs; changed camera angle for rollers when splashing back to Version 1.3; changing scoring of close Tower Control games back to Version 1.3; slightly changed Arowana Mall and Camp Triggerfish)
Sept. 4: Added: Mini Splatling
Sept. 11: Added: H-3 Nozzlenose
Sept. 17: Added: Hammerhead Bridge
Sept. 19: Fifth Splatfest (art vs. science)
Sept. 25: Added: Tri-Slosher
Oct. 2: Added: Carbon Roller Deco
Oct. 10: Added: Custom E-liter 3k Scope, Custom Range Blaster
Oct. 10: Sixth Splatfest (cars vs. planes)
Oct. 20: Patch Version 2.2 (fixed bugs; nerfed indirect damage for all blasters and rollers; decreased ink usage for Splattershot Pro and Squelchers; adjusted ranking calculation; adjusted rules for Rainmaker and Tower Control; adjusted Walleye Warehouse and Kelp Dome in Rainmaker; adjusted Saltspray Rig, Arowana Mall, Flounder Heights and Hammerhead Bridge in all modes; made position-tracking for teammates less visible; made Inkzooka louder)
Oct. 29: Added: H3 Nozzlenose D, Luna Blaster Neo
Oct: 31: Seventh Splatfest (pirates vs. ninjas)
Nov. 6: Added: Heavy Splatling Deco
Nov. 10: Added: Neo Sploosh-o-matic
Nov. 12: Patch Version 2.3 (fixed bugs; added 42 new pieces of gear in shops; changed Ranked Battle results screen to show players in the order of their winning percentage over the previous 10 games rather than in the order of splats in that game)
Nov. 13: Added: Museum d'Alfonsino
Nov. 17: Added: Bamboozler 14 Mk II
Nov. 20: Added: Hydra Splatling
Nov. 24: Added: Slosher Deco
Nov. 27: Added: Sloshing Machine
Dec. 3: Added: Mahi Mahi Resort
Dec. 5: Eighth Splatfest (burgers vs. pizza)
Dec. 11: Added: Zink Mini Splatling
Dec. 17: Patch Version 2.4 (fixed bugs; players of similar skill matched more often in Turf War and Splatfest; added prohibited areas in some Rainmaker maps; shortcut paths removed from Mall, Port, Heights and Museum)
Dec. 18: Added: Tri-Slosher Nouveau
Dec. 19: Ninth Splatfest (naughty vs. nice)
Dec. 25: Rapid Blaster Pro Deco
Dec. 28: Added: Piranha Pit
It feels like updates have picked up a bit... maybe to make people who just picked it up since Black Friday feel like early adopters did in the first few months.