Sometimes day 4 or 5. All you'll see that early though is little streaks of veins if you know what to look for and not seeing anything won't mean anything. You are best waiting until day 7-10 if you haven't candled before or if you want to make any decisions about the eggs. If you just can't wait to see what's going on though you can candle day 4-5 and again day 7-10. Irregardless of when or how often I candle I usually mark potentially bad eggs on day 7 and then 2-4days later if they still look bad throw them out.