First I'll answer something you did not ask but it might be handy to know when making your plans. It takes about 25 hours for an egg to go through a hen's internal egg making factory. That egg can only be fertilized in the first few minutes of that journey. That means if a mating takes place on a Monday, Monday's egg will not be fertile from that mating. Tuesday's egg might or might not be, depending on timing. Don't count on it. Wednesday's egg will be fertile.
Note that this is after a mating. A rooster doesn't necessarily mate with every hen in his flock every day. But he doesn't have to. In the last part of the mating act the rooster hops off. The hen stands up, fluffs up, and shakes. This fluffy shake moves the sperm to a special container near where the egg starts its journey. The sperm can remain viable in that container anywhere from 9 days to over three weeks. So he only has to mate with a hen less than once a week to keep her laying fertile eggs.
This is where the uncertainty comes in. You can't tell for sure when that sperm becomes non-viable. Many people wait three weeks before saving eggs to hatch. That usually works. But to be absolutely sure four weeks is better. It depends on how sure you need to be.
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