When Should Sysron Be Taken To Prepone Periods?

Hie doctor... I hav been prescribed sysron n tabs fr 3 days into 3 tabs daily... I want to prepone my periods... My actual date is 23rd feb but I want to get my menses by 20th feb... So from wen should I start de course n wil I get my periods by 20th as I'm goin out of station for religious ceremony...