One of the many Trophies / Achievements in Red Dead Redemption 2 (RDR2) is the ‘Marco Dragic’ (Artificial Intelligence) one. Which requires you to complete two separate quests one of which is only doable during a specific weather condition.
To begin this quest you will firstly need to of reached Chapter 4. Whilst in Chapter 4 you then need to head for Saint Denis. Now for the important part, most if not all of the quests that are required for the Achievement is the condition of the Weather. For example to start the first part of the mission, it will only activate during the DAY time, meaning if its currently nightfall it will not activate. So head for Saint Denis and then to the pond/lake area which should be indicated on your Map.