Time Machine Week 1 Back Up Mystery

I was hoping someone could help me better understand the rules Time Machine uses when keeping weekly backups.

Specifically , I am looking now at my TIME MACHINE which began its first back up a year ago, I am trying to work out if the first first back on my Time Machine (ie week 1), contains an exat copy of my Mac as it was when I first ran Time Machine for the first time (eg hour 1 on day 1). or is it an exact copy of the state my Mac was in at the end of that first week of using my Mac?

So in otherwords - when TIME MACHINE saves a full week, is it taking a snaphot of the Mac from the start of or end of that week.

Does anyone know?