What time does the business with ID no.12 open on Monday?
open time refers to opening_time; on Monday refers to day_of_week = 'Monday'; business with ID no. refers to business_id 


SELECT T1.opening_time FROM Business_Hours AS T1 INNER JOIN Days AS T2 ON T1.day_id = T2.day_id WHERE T1.business_id = 12 AND T2.day_of_week = 'Monday'