JR Kanto Pass

Hi Hi,

I am traveling to Japan in December for 5 days. I am wondering how does the Kanto Pass work? It said it is valid for consecutive 3 days so if my first use is on Monday 1pm, will the pass expire on Thursday 1pm?? Or it will expire Wednesday 11:59pm??

And also do i have to use the pass first in order to reserve a seat? I know on Wed and Thursday, I am traveling to Kusatsu Onsen, when is the earliest time I can reserve a seat?

thanks for your help!