Yeah, sounds like you turned two pages on your calendar instead of one. Ouch!
I'll ask a silly question, do you know the date the eggs were set? Obviously not since you are asking your question but I'll ask anyway.
What method did you use to originally calculate the date? Some people count days on the calendar (1, 2, 3, .......) until they get to 21, turning pages as necessary.
Some may use math and add 21 days to set date to come up with a date. Say you set on the 12th, add 21 to 12 and come up with 33. Since March has 31 days, subtract 31 and come up with the 2nd.
I know the day of the week I set them is the day of the week the 21 days are up. If I set on a Friday the 21 days are up on a Friday so I count down three weeks on that day of the week.
Can you back calculate using your method skipping April entirely. That should give you a date to work with.
You can then confirm that by using Sussex's method. You can find photos online or in the "Articles" section of this forum of what you should see each day when candling. Or post photos on here and get people to help you guess. This will not give you an exact date but it will confirm if your back calculation is in the ballpark or not. And it may be the best you can do.
