Where I work at least, your shift is all overtime if
a) It's outside your normal work week. Which in this case monday is outside your normal work week.
b) It's your 6th shift inside your normal work week. Which means you'd have to take an extra shift inside the tuesday through saturday period.
Working outside your normal work week also doesn't count towards your 6th shift as it's already overtime and you'd be double threating at that point. However, you also don't have to work all the shifts in your work week to get that overtime either. So for me, my scheduled days are mon-fri. I can work sunday, miss the entire work week after that, and still get overtime for my sunday shift.