When is day 18 and day 21?

[slickchik]

I have hatched three batches of eggs and I have set them all around 7pm or so.

For my new batch, I did it differently and set them at 8am Saturday the 10th...

I can't decide when exactly day 18 is and when day 21 is...

When should I expect them to start hatching?

HELP!

 

[equine chick]

day 18 is 28th at 8am