A colloquial expression used when trying to recall or confirm something ('was it...? / remind me...').
彼、今日来るんだっけ？
Is he coming today? (Was that right?)
あの店、駅の近くだったっけ？
Was that shop near the station?
私、明日休みだっけ？誰か教えて。
Am I off tomorrow? Can someone tell me?
名前が出てこない…あの人、何て言ったっけ？
I can't recall the name… what did that person say again?
Used to express nostalgic recollection — 'we used to...' or 'I remember (it was)...', often recalling past states or habits.
子どもの頃は毎日ここで遊んだんだっけね。
We used to play here every day when we were kids, didn't we?
昔はよくこの店に来たんだっけ、懐かしいな。
I used to come to this shop a lot — how nostalgic.