Update 10/03/2022 the description has been updated now, to indicate that everything within a festival playlist series (I.e everything listed under summer, autumn, winter and spring) must be completed to obtain this achievement.
UPDATE 05/12/2021: as I suspected at first, it seems that every single activity must be completed in each of the 4 seasons of a festival playlist series, which mirrors the Encore achievement from FH4. This should not come as a surprise. There have been reports of people unlocking it at 247 points (pre December update which fixed online issues), but this seems to be a random few lucky individuals. In addition, a recent update was released which should make completing the Horizon Arcade much easier - requirements will now scale to the number of players participating in the Arcade and there should be more large groups tackling the challenges. If someone can confirm that they have found it easier to complete the Arcades, that would be appreciated.
Note, the description of the achievement on the Xbox is "Complete all Seasonal Championships in a Festival Playlist Series", which is incorrect.
Series 1 begins on 11th November, so this shouldn't be unlockable until 4 weeks after this date (sometime in December) at the earliest, as a Series refers to the four seasons each lasting 1 week (with the welcome week not counting as part of a series) starting with summer.
Then you have to earn 247 activities in the festival playlist (although it may be slightly glitched). You must earn every single point within the Festival Playlist Series, much like in FH4. Also worth noting that this must be done in a single Festival Playlist Series, so you can't do Winter and Spring from Series 1 and then Summer and Autumn from Series 2 - all 4 seasons must be completed within the same Series.
FYI - the Trial only unlocks upon reaching the Horizon Hall of Fame. This requires you to earn 200k accolade points.
Also, beware of the arcade activity! It's extremely difficult to find random players to do these with and they're impossible to complete with 4 or fewer players. I have also had problems finding an Eliminator match (which I hate anyway) but again it's needed for the Festival Playlist Series.
To those who downvoted, I apologise, but the achievement was bugged and attainable earlier than was probably intended, and there was conflicting info between TA's description and the Xbox description.