import datetime  
def every_20_days(date):  
    print('Starting Date: {d}'.format(d=date))  
    print("Next 12 days :")  
    for _ in range(12):  
        date=date+datetime.timedelta(days=20)  
        print('{d}'.format(d=date))  
  
dt = datetime.date(2016,8,1)  
every_20_days(dt)