Here are all the Forzathons and achievements that is planned for October. I've included the last one in September since it runs into October:
Take the Lead (Sept 29 – Oct 4)
- Forza Series: Complete a Championship in a Porsche to win 3 Wheelspins and 55,000 CR.
- Hurrying Up: Perform 30 Speed Skills to receive 80,000 CR.
- That's Gotta Sting: Win a Race in an Abarth to earn 95,000 XP.
- Leaps and Bounds: Get 3 Stars on the Airport Danger Sign to win the Porsche 911 Turbo 3.3 and 35,000 XP.
Speed is Key (Oct 6–11)
- Flare-lit Drive: Complete a Street Race in a Hypercar to earn 55,000 XP.
- Go Faster: Perform 20 Speed Skills to win 3 Wheelspins.
- Hyper Fast: Win a Race in a Hypercar to win 80,000 CR.
- Release the Beast: Get 3 Stars on a Speed Trap in a Hypercar to win the Jaguar F-Type Horizon Edition.
Chase Down Victory (Oct 13–18)
- Old School: Complete 3 Races in a Retro Muscle car to win 3 Wheelspins and 35,000 XP.
- Armchair Enthusiast: Complete a Championship to receive 120,000 CR.
- Blindspot: Perform 5 Pass Skills to earn 35,000 XP.
- Ten Eighty in Progress: Win a Street Race in the Crown Victoria to receive the Porsche #45 Flying Lizard GT3 RSR.
Vision Becomes Reality (Oct 20–25)
- Speed Test: Complete 5 Head-to-Head Races to win 3 Wheelspins and 35,000 XP.
- Showing Your Best: Perform 5 Show Off Skills in a Porsche to earn 85,000 XP.
- Rear Engine Domination: Get 3 Stars on a Speed Trap in a Porsche to receive 120,000 CR.
- Unleashed Potential: Create a Bucket List in a Porsche to win the Porsche 959.
We're Counting on You (Oct 27–30)
- Waiting for Delivery: Win a Championship in a Cult Classic to receive 85,000 CR.
- Another Round: Complete 3 Playground Games to win 3 Wheelspins.
- Working Hard: Perform 20 Drift Skills in a Mercedes-Benz to earn 35,000 XP.
- Don't Let Us Down: Get 3 Stars on the Mountain Top Danger Sign in a Mercedes-Benz to win the Mercedes-Benz 300 SEL 6.3.
Horizon Halloween (Oct 31, 1 day only!)
- Survival: Survive a Public game of Infected to win 3 Wheelspins and the Dodge Dart Horizon Edition.
- Infernal Skills: Perform 5 Daredevil Skills to receive 120,000 CR.
- Show Me: Complete a Street Race in the 1958 Plymouth Fury to earn 65,000 XP.
- Venomous: Complete a game of Infected in a SRT Viper GTS to receive 225,000 CR and the Porsche 918 Spyder.
Edit September 7: Added all achievements for October.