Hey there!

You can apply for it a month before the eligibility period if you want. However, you are advised to visit the following links and make sure you have gone through the information given carefully, and make sure that you have all the required documents whilst applying:

https://iwa2.ecfmg.org/appforcert/onlinedocs/afcoverview.aspx

https://iwa2.ecfmg.org/overview.asp

https://iwa2.ecfmg.org/IwaFAQ.asp#14

https://www.ecfmg.org/resources/publications.html#boi

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

Hope this helps. All the best!