You're going to need to provide more information...
Specifically, how do you define a full week??
Is it simply from Sunday to Saturday
Or Monday to Sunday
Or Monday to Friday
?
What exactly do you want to do once the last full week is found?
do you just want a list of all 7 dates within that week?
Assuming you mean a full week is Sunday to Saturday..
And you mean of the Current year..
Then put this in a cell, say A1
=DATE(YEAR(TODAY())+1,1,MATCH(7,WEEKDAY(DATE(YEAR(TODAY())+1,1,{1,2,3,4,5,6,7})),0))-13
That gives you the Sunday of the last full week
Then in A2 put
=A1+1
And fill down to A7
Hope that helps.