Treasure Hunts are located in the "Challenges" category on the right side of the Festival Playlist menu and are titled "Treasure Hunt". You will have to do a challenge that once you complete it will mark an area on the map (similar to a barn find) in which you'll need to look for the Treasure Chest. Note: if you have purchased the Treasure Map DLC, the spot will be correctly marked immediately.
In Series 1 there are two Treasure Hunts the first is in Summer (11/Nov/21 - 18/Nov/21) and the second is in Spring (02/Dec/21 - 9/Dec/21) Note: exact timings may be different due to timezones. Retrieve both Treasure Chests and you unlock the achievement.
The first day this is possible to unlock is therefore December 2nd. People who have already unlocked this had early review copies to the game. In future series there may be more/less Treasure hunts per series.
Below is an example image of what a Treasure Hunt looks like on the Festival playlist (Example is taken from Series 1 - Summer).
