It takes about 25 hours for an egg to make its way through a hen's internal egg making factory. That egg can only be fertilized during the first 15 minutes or so of that journey. So if a mating takes place on a Sunday, Sunday's egg is not fertile. Monday's egg might be but that depends on what time of day the mating took place and what time the egg started its journey. Tuesday's egg is almost certainly fertile. This is after a mating. A rooster does not necessarily mate with each hen in the flock every day, but if you have only one or two hens with him, it will probably happen.
