Hey there!

The date has to be chosen within the eligibility period (which is for 3 months) after ECFMG certification. For more information regarding the step 1, step 2 and step 3, the dates and the eligibility criteria, you are advised to visit the following sites:

https://www.usmle.org/

https://www.usmle.org/bulletin/eligibility/

https://studyabroad.careers360.com/articles/usmle

https://studyabroad.careers360.com/articles/usmle-step-1

Hope this helps. All the best!