If you set them on Sunday, April 18, then the same time of day you set them on Sunday May 9 is when the 21 days of development is up. Then that same time of day on Thursday, May 6 is when you should go into lockdown since that is the time that 18 full days of development is reached. It is not an exact science and if you are off several hours, it is not a big deal.
It can be confusing. It helps me to think if it that they have one full day of development after 24 hours of development, not 1 second after I put them in the incubator. Lockdown should come after 18 days of development, not after 17 days and 1 second of development. And the day of the week that you set them is the day of the week that the 21 days is up.
I'm sure you realize that all your eggs will not hatch exactly 21 days after they started. Hatching is a process. They pip. They rest, sometimes a long time. They zip. They gather strength and push out of the egg shell. They are wet and very tired. They rest. Eventually they get up and start knocking the other eggs all over the place. They eventually dry off. Others can be at various stages of this process.
Also, if your average incubation temperature is a tad high, they can hatch a day or two early. If it is just a fraction low, they can be a day or two late. It is a worrying time, but very rewarding when it is over.
Good luck!!! We do expect photos.