u can register any tyime of the year.u have to register on the ADA website , ada.org, go to education and testing and then click on NBDE part 1 , there u will find a link to register.U can fill out the online application and pay using a credit card. u have to ask ece to send them your evaluation report, directly.
once the ada approves your application, you will get a letter from them and a number ,which u will then use to take the exact date and time of the test.U can do that on the site prometric.com or they will also give u the phone number of prometric. u can do this only after u get the confirmation number from ADA.
hope this helps.This is the procedure for graduates of foreign dental schools, i am assuming you are one .
I registered for it w hile ago and am relying on my memory ! so if thers any mistakes here, pls feel free to correct me.thanks.