〜っけ
was it? (trying to remember)
Plain past / だった + っけ · ましたっけ · でしたっけ
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っけ turns a question into "remind me…" or "was it…?": 会議は何時からだったっけ, "what time was the meeting again?" You're searching your memory, and half asking the listener, half asking yourself.
It usually follows a past form, because you're checking something you once knew: だったっけ, 置いたっけ. The polite versions are ましたっけ and でしたっけ: 前にも話しましたっけ, "did I tell you this before?"
Compare よね, which checks that the listener agrees with something you're fairly sure of. っけ admits you've forgotten.
Talking to yourself, it's a mutter: 鍵、どこに置いたっけ, "where did I put my keys?"
Register: Casual, but ましたっけ and でしたっけ are fine in polite speech.
Examples
会議は何時からだったっけ。
What time was the meeting again?
あの人の名前、何だったっけ?
What was that person's name again?
明日は休みだったっけ?
Wait, is tomorrow a day off?
鍵、どこに置いたっけ。
Now where did I put my keys?
前にも話しましたっけ。
Did I tell you this before?