Airport Expeess travel pass

If I'm buying the 2-rides AE;-

Sunday- AE ride
Does the 3 consecutive days start counting from Sunday itself & valid till Tues or it'll only start counting when I use it for my 1st MTR rides?

I'll be spending a 5D4N in HK from 11/9-15/9 and I basically nid 2 rides which is on the 11/9 & 15/9 respectively..