Blizzard is normally pretty consistent with the dates they release expansion.
There's also a announcement 2-3 weeks ahead of time when there is new content coming up.

Darkmoon Races

released 21st of januari

Onyxia's Lair

released 15th of februari

As of now there has been zero news about the coming mini-set. Or am I missing something?

Hopefully - never, so we can save 2000 more gold for new packs in April :)
Mini-sets where kind of boring and low impact content, introduced just to strip the players of gold, mid expansions =(

Quote from AAZ >>

Mini-sets where kind of boring and low impact content...

Are you looking at the same cards as I am?

Darkmoon Faire: Luckysoul Hoarder, Illidari Studies, Arbor Up, Guidance, Imprisoned Phoenix, Conjure Mana Biscuit, Hysteria, Nitroboost Poison, Backfire, Armor Vendor, Crabrider

Barrens: Lady Anacondra, Shroud of Concealment, Primal Dungeoneer, Wailing Vapor, Stealer of Souls, Unstable Shadow Blast, Mutanus the Devourer

Stormwind: Jerry Rig Carpenter, Moonlit Guidance, Druid of the Reef, Doggie Biscuit, Deepwater Evoker, Grey Sage Parrot, Amulet of Undying, Edwin, Defias Kingpin, Blackwater Cutlass, Cookie the Cook, Brilliant Macaw, Shadowblade Slinger, Wicked Shipment, Defias Cannoneer, Mr. Smite, Multicaster

AV: Scale of Onyxia, Dragonbane Shot, Pet Collector, Furious Howl, Drakefire Amulet, Smokescreen, Tooth of Nefarian, Raid Boss Onyxia, Kazakusan

Sunken City: Fossil Fanatic, Herald of Nature, Ancient Krakenbane, Shellshot, K9-0tron, Lightray, Shattershambler, Jackpot!, Command of Neptulon, Clownfish, Tidal Revenant, Neptulon the Tidehunter

Nathria: Objection!, Order in the Court, Class Action Lawyer, Scribbling Stenographer, Perjury, The Jailer, Sylvanas, the Accused

Sure, the latter ones are less impactful but they aren't around as long.

The miniset is probably announced soon and potentially late February.

Miniset content doesn't take 2-3 weeks to launch after the announcement, it might take a week at most, let say if they announce on this Thu, Feb 2, it might be released on Feb 7 or Feb 9. 

Anyway, it should be coming soon in early Feb if they want to give the next meta for 2 months and keep up with their annual Core Set rotation along with Spring expansion to launch around early-mid April. 

It'll be announced ~1 week after Heroic Brawl starts tomorrow

Onyxia's Lair was announced on Thursday the 10th of February last year, dropping just five days later on a Tuesday. If Blizz follows a similar schedule this year, the announcement will be on the 9th of February with the set dropping on the 14th. 

As is always the case, they announce the mini-set immediately after the Heroic Brawl. You won't hear anything until Wednesday, February 8th. This is so people waste their gold on heroic brawl, and then have to spend real money on the mini set if they want it. You don't ever have to ask "when is the mini set going to be," because the answer is always immediately after the next heroic brawl.

Quote from Speckix114 >>

As is always the case, they announce the mini-set immediately after the Heroic Brawl. You won't hear anything until Wednesday, February 8th. This is so people waste their gold on heroic brawl, and then have to spend real money on the mini set if they want it. You don't ever have to ask "when is the mini set going to be," because the answer is always immediately after the next heroic brawl.

 Finally, someone gets it.

This is also why the release has started to happen the day after the announcement. It gives you less time to accumulate the gold you'll need for the purchase. (Note that the easy solution is to avoid Heroic Brawl like the plague.)