I have a rooster and I understand it only takes a day
Let's look at it this way. An egg takes on average 25 hours to make its way through the hen's internal egg making factory. It can only be fertilized during the first few minutes of that journey. Any egg laid that day cannot be fertilized by that mating. An egg laid the following day might or might not, depending on timing. I don't count on it. An egg laid the next day should be fertile.
A rooster does not necessarily mate with every hen in his flock every day but he doesn't have to. The hen stores the sperm in a special "container" near where the egg starts its journey. That sperm can stay viable for as long as three weeks but that is stretching it. I'd count on two weeks.
but do the hens need to get used to laying first before the eggs are viable?
When they start to lay, pullets can have issues putting an egg together correctly. Different things can go wrong: soft shelled, no shell, double yolked, no yolk, no whites, or something just plain weird. It can take a few days for the pullet to straighten this out.
Most do actually get it right from the start but there is another problem. The first eggs are typically really small. There is not enough nutrients in them for the chick to grow to a normal size. There is also not enough room in there for it to grow very big. I'm not talking about the difference in size of a bantam egg versus a full sized hen egg. I'm talking about the small egg compared to the size that hen will eventually lay.
I've hatched some first-time pullet eggs. The hatch rate might even be decent but it is often not as good as if you used larger eggs. Most chicks that hatch live and thrive, but if I have a chick die after hatch it is more likely to have come from a small egg.
I've found if I wait until the pullet has been laying at least a month these problems pretty much go away.
And if anyone has used this incubator before how is it? It's labeled WYD2ANG but the instruction manual says 'Square Grid M12'.
I'm not familiar with that specific incubator but a lot of chicks have been hatched in similar designs.
Good luck!