“When is it again?”
You were invited to a cousin's wedding. Your sister asks you if you're going. You don't remember when the wedding is, so you ask like this.
When is it again?
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"When is it?" is a normal way to ask when something is going to happen.
A: Do you think you can make it out to my art show?
B: Um, when is it?
A: It's next Saturday, in the afternoon.
If someone told you when something was going to happen, but you forgot, you could also ask:
When was it?
You can ask this way, even if the event hasn't happened yet.
Use this phrase when you can't remember something that you're supposed to remember. For example:
I'm sorry. What was your name again?
How old is your daughter again?
What was the password again?
We use this expression mostly in spoken English. It's not common in formal writing, prepared speeches, and so on.