[Update]: After a few delays, MTG Arena's Historic Horizons expansion has now been unleashed!
If you were hoping to get your hands on some of the 372 new-to-Arena cards this August 12th, I'm afraid I have some bad news to deliver. According to the latest developer update, Jumpstart: Historic Horizons has been delayed until August 26th, which also means that the free two-color "New Player Experience" decks have been pushed back as well.
While that certainly does suck, you'll be happy to hear that the time should go by extremely quickly as Wizards of the Coast is planning to run a variety of events until Historic Horizons' launch, as well as give away some freebies as a little consolation prize. And yes, the plural there was intentional! Your first entry into the upcoming Amonkhet Remastered (August 10-15), Kaladesh Remastered (August 15-20) and Ikoria: Lair of Behemoths (August 20–26) Premier Drafts will be entirely free of charge, and you will indeed get to keep all of the card, gem and pack rewards!
The good news doesn't end there, however, as 100-card Historic Brawl will be arriving earlier than expected in order to help tide everyone over. The completely free Historic Brawl event will be running from August 12-24, after which it will get replaced by the (hopefully) permanent variant. The exact fate of Historic Brawl will heavily depend on how popular it remains throughout the next few weeks, so if you're a fan of the format make sure to actually play it so it sticks around for good.
It's also worth mentioning that the August ranked season will reward twice as many cards and gold coins than normal. So whether you care about ranked play or not, I'd highly recommend playing at least a few games as you'll get a massive payout for very little effort.
You can read a little bit more about this delay, as well as Historic Horizons in general, over at the MTG website. And just for good measure, I'll leave you with a brief schedule of the upcoming MTG Arena events:
• 'First One on Us' Historic Premier Drafts: August 10–26
• No Entry Fee Historic Brawl Event: August 12–24
• MTG Arena Game Update: August 24
• Jumpstart: Historic Horizons: August 26–October 14
• August 2021 Ranked Season (with combined pack and style rewards): Now–August 24
• September 2021 Ranked Season: August 24–September 30
• Innistrad: Midnight Hunt Release: September 16