The full sentence is
I want to know this 一週間前までに meaning. I tried thinking and came up with "within one week before (the deadline which is not mentioned)" but I'm not sure if it's correct.
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Let's look at the different parts:
Putting it all together:
For those wishing to watch [a training session / a lesson], please get in touch with the contact person at least a week in advance.
(You will have to adapt the translation of 見学 depending on the context.)