The sentence:
This household has been in a state of excitement over the trip since weeks before it began, when we said, "In one month, you'll be in London!"

What does "when" refer to? It seems like it refers to the entire sentence in front, just as the non-restrictive usage of "which" sometime does. But is such usage possible for "when" as well?

Or maybe it refers to "weeks before it began"....I don't know. I have no idea.

I need your help.
Hi Taka

My guess is the 'when' refers to 'weeks before it(=the trip) began'.

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I agree with Paco.
Thank you!