JR pass validity and seats reservation

Hi there,

I'm flying into Osaka on 24 Dec, will be staying in Kansai area until 2 Jan. And then we are going to travel by train to Tokyo on 2 Jan and return to Osaka on the 7th. We have bought a 7-day JR pass to use for this part of travel (2 Jan to 8 Jan), I will be leaving from Osaka on the 8th Jan.

Question is, can we get our JR pass once we arrive in Kansai airport, but not validate it until 2 Jan, but still be able to reserve a seat for that day (2 Jan) when we first arrive at Kansai airport? Or do we have to validate our pass straight away (this case on 24 Dec) in order to make any seat reservations for a future date?

Thanks,
Wanda