At what year of medical school can i get usce

Hi
i was wondering which summer vacation can i use to get the letter of reccomendation ( is this given every time someone do a usce ?) and i don't know if i'm too young !

Which summer is best ?

1 year , 2 y , 3y , 4y ,5y or 6y ( but in this case i have the final proof on july and i'll became IMG )

Thanks