On this page of the Disco Elysium guide, you will find an accurate description of the passage of the side task Ask Claasje about Sunday night.
In the Ask Claasje about Sunday night task in Disco Elysium, you should talk to Klaasje about what went on with Harrier over the course of Sunday night.
The task can be received earlier, but can only be done when Harry is left alone, such as when Kim goes to his room or goes to the station to transport the body.
You can receive the task during the first interview with Klaasje on the balcony if you ask where she was on Sunday night: Were you in Sunday night? I need to know what I did before I lost my memory.
Go back to Klaasje when you're left alone - then you can talk to her about Sunday night (Now that Kim isn't here - let's talk about Sunday night ). The woman will tell you everything in detail and the task will come to an end.