Your friend has an important week coming up, but you cannot remember whether it is an interview, a presentation, or the first day of a new job. Forgetting a detail does not stop you from being interested. Ask rather than bluffing your way through.
Stay with the thing they are telling you
If your attention is split, put the other task aside when you can. Respond to what they said before moving on: “That sounds like a lot to take on.” Then ask what part is worrying them, or whether they want to tell you more.
Remembering a date and understanding why it matters are different jobs. A memory aid can help with the first; listening gives your friend room to explain the second. You do not need to turn the conversation into a series of questions.
Ask again without making it a big event
“You had something coming up at work. Remind me what day it is?” is an honest question. Listen to the answer instead of spending the next few minutes explaining how terrible your memory is.
Check whether an old plan still applies, too. Asking “Did you end up taking that class?” leaves room for a changed mind. Assuming they went can make an ordinary catch-up unnecessarily awkward.
Follow their interest
If you intend to ask how an interview went, add a reminder for afterward. Experiments on remembering intended actions support using external cues. Keep whatever helps you, without saving sensitive details you do not need.
Next time, ask about something they seemed happy to discuss. If their answer is brief or they change the subject, follow their lead. Remembering a detail is a way to open a conversation, not a reason they owe you an update.