One can attend student visa interview within 120 days from start date mentioned on form I-20.
If start date on I-20 is august 18th 2014, you can attend visa interview from April 19th 2014. You are not supposed apply for visa appointment with US consulate before 120 days period. Typically start date on I-20 will be 3rd Monday of August for Fall semester and 3rd Monday of January for Spring semester.
June and July months very busy seasons of year for student visas since Fall semester student will be attending for visas. November, December are peak seasons for Spring semester students. Most of the students delay to attend visa interview till they hear from all the universities they have applied. But the good thing is to attend for visa interview as soon as you get the I-20 from your favorite university.
Idle time for Spring 2015 students to attend visa interview is October or November 2014.
Book visa dates ahead at least a month before. Spring 2015 students should start preparing required documents for F1 Visa interview such as bank statement, affidavits, assents evaluation.
Don’t wait till you receive final degree certificate. You can also visa interview with out final degree certificate.