It takes an egg about 25 hours to make its way through a hen's internal egg laying factory. The egg can only be fertilized during the first 15 minutes or so of that journey.
If the mating takes place in a Monday, Monday's egg is not fertile. It started its journey on Sunday. Depending on what time the mating took place and the egg started its journey, Tuesday's egg might be fertile. But it might not. Wednesday's egg is almost certainly fertile.
Notice that this is after a mating. A rooster does not always mate with each and every hen in the flock every day. Usuallyy a hen will stay fertile about 2 weeks after a mating, sometimes a little less but usually a little more.
This link tells you how to look for the bull's eye. Good luck!
I know this is an old thread, but... my broody just hatched 4 serama eggs for me. Three were given to me by the breeder of the hen I got on march 10, those were set on 3/12. The 4th was laid on 3/13 and set the same day. I just assumed it would also be fertilized by the roo she had been kept with, but it is almost all black with dark legs... like MY roo. The other 3 are shades of blonde with yellow feet. Could my roo be the daddy after 2 days?