Half board meal plan question

Sorry I know there are a million of these but it’s our first time visiting and I’m confused on the logistics of the half board meal plan.

It says you get 2 dematerialised meal vouchers for every night you’re there - 1 breakfast, 1 lunch or dinner.

So far so good…

But I’m confused as to when they are actually valid

We arrive mid afternoon on the Friday (leave on the Monday), so do we have to use our first lunch/dinner voucher that day or could we save it for the last day?

I’m just trying to get organised to book tables so want to make sure I’ve got them booked on the right days