Please indicate the opening day of businesses whose category is pets.
category is pets refers to category_name = 'Pets'; opening day refers to day_id from Business_Hours and opening_time;
SELECT DISTINCT T4.day_of_week FROM Business_Categories AS T1 INNER JOIN Categories AS T2 ON T1.category_id = T2.category_id INNER JOIN Business_Hours AS T3 ON T1.business_id = T3.business_id INNER JOIN Days AS T4 ON T3.day_id = T4.day_id WHERE T2.category_name = 'Pets'