Hey there!

You can only appear for step 1 of USMLE only during the eligibility period, that is for 3 months, after you have got your ECFMG certification. For more information regarding the USMLE step1, step 2 and step 3 exams, ECFMG certification and it's dates, you are advised to visit the following sites:

https://www.usmle.org/

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

https://www.ecfmg.org/

Hope this helps. All the best!