Adding this solution because it appears on TA's "Guideless Achievement" list - and because there were a couple of hiccups I had while getting this achievement and I thought I might share them with the (probably) small percentage of the population who experienced the same hiccups.
So, first things first, where
to find the list of #FORZATHON activities:
they can be found in many different ways:
1. When you start Forza Horizon 4 it now takes you to the "Festival Playlist" - two of the tiles will display the "Weekly #FORZATHON CHALLENGE" and the "Daily Challenges" - you can click on either of those tiles to view the challenges in more detail.
2. From the "Welcome Back" screen, there is a tile on the left labeled "#FORZATHON" that you can click on to view the challenges in more detail.
3. Anytime while exploring/driving/etc, you can hit the
button to bring up the menu, then
to the "Horizon Life" tab and click on the "#FORZATHON" tile and view the challenges in more detail.
There are many, many other ways to navigate to the list of #FORZATHON challenges, but those are the three ways I most often use.
So, next, when do the Weekly challenges start:
Seasons change every Thursday at 10:30 EDT/2:30 PM GMT - and thus the #FORZATHON weekly challenges begin at this time.
The Daily challenges update every day at the same time listed above.
Why does this matter? Because you have to do 7 daily challenges in one week, so you can't start on say, Monday, because you'll have (at most) only 5 daily challenges available for the week at that point and you won't be able to get the achievement.
This leads to my discussing some of the hiccups you might experience while trying to get this achievement.
My first bit of advice is DO NOT
complete the last two daily challenges from the previous week. If you take this advice, then instead of having 7 daily challenges available for the week (one every day at 10:30 EDT), you will have 9 available - and thus you can skip (or miss) 2 and still complete the achievement. Sometimes the daily achievements are annoying and having the opportunity to skip one that you particularly hate is advantageous. (For example, one of the daily challenges right now is "Shake N' Bake - Combine Drafting and Pass Skills to earn 2 Slingshot Skills" - maybe you hate having to do that particular task, so you skip it and replace it with an easier task from the extra two daily challenges you have available.)
Which leads to the second piece of advice (and my first major hiccup when completing this achievement): actually look
at the Weekly challenges before deciding to complete the achievement. The Weekly challenges include 4 chapters which must
be completed in order
(this was another hiccup I ran into, as I didn't realize at first that they had to be completed in order, I couldn't complete the Chapter 3 or 4 objectives until I had completed Chapter 1, then Chapter 2 - don't be like me and spend an hour trying to accomplish Chapter 3 tasks when you have completed, nor even attempted the tasks associated with the first two chapters).
The first Chapter is usually just drive the specified vehicle - it might be a specific vehicle or a car class or a particular car type. However, if you don't own the specific vehicle you might have to purchase it. I did this on my first go round at this achievement, and I even looked at the list of chapter objectives before I purchased the rather expensive specifically required vehicle, and then I got to Chapter 4. Chapter 4 that particular week was complete (or win) a "Goliath" race (which requires reaching Level 20 in Road Racing). I hadn't reached Level 20 yet - and was actually quite a ways from getting to that level - so I didn't complete the Weekly challenge list - and thus I wasted the serious coin I spent on the required vehicle - and the time required to complete the previous chapters and daily challenges.
So, again, look at the weekly list and then make the determination if it's worth the coin to buy a vehicle (assuming you don't already own one that meets the requirements) and/or if one of the challenges is going to be a pain to complete or will require a time commitment you don't currently have. If there are any issues, wait a week. Invariably there will be weeks with trivial challenges that are super easy to accomplish with minimal effort.
Finally as a bit of FYI, to view each of the Weekly challenges once you are in the #FORZATHON menu, simply click on the "Weekly Challenge" tile and then use the
stick to navigate to each chapter (this isn't obvious when you are in the menu).
The Daily challenges are listed on a separate tile (there are only three daily challenges available at any time - and they are each only available for 3 days ).
All the challenges have countdown timers, so you know exactly how much time is available before they expire - and each challenge has a counter in the bottom right corner displaying the count requirement and how many of that particular task you have already completed.
Overall this is a really easy achievement to complete (despite the current high TA ratio) - it's only difficulty lies in the possible extended time commitment and the necessity of returning periodically to complete the challenges.
Although you can really do it in just 3 days: Thursday after the Weekly challenges reset, knock out all the weekly challenges and three daily challenges (assuming you saved two from the previous week), then return on Sunday after the daily reset and knock out the next three daily challenges, then return one final time on either Monday, Tuesday or Wednesday and knock out the final daily challenge: ACHIEVEMENT COMPLETE!!