The word you use when you are talking about something that happened in the recent past, but you can't remember the exact day (due to a black out), or it really doesn't have any significance to the story...or you just can't be bothered to think hard enough to remember it.
Amber: I just went to a Ween concert in Anaheim.
Penny: Really, when was that?
Amber: Lasterday, I think.
Penny: Really, when was that?
Amber: Lasterday, I think.
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