Better to activate it on the day : you could be ill, there could be cancellations, strikes, etc. (hopefully nothing happens of course!)

Once the validity period or a pass day has started you cannot change.

No such thing as a silly question! Please ask - it’s better to find out now that make a mistake!

As @thibcabe says, it’s best to wait until the day of travel before activating your pass - there’s no benefit to doing it now. If you do it in advance, there’s always the risk that plans may change.

You can still do all your planning, and make any reservations you need, without an active pass.