Korean Particles(조사) are divided into "Case particles(격조사)", "Conjunctive particles(접속조사)" and "Auxiliary particles(보조사)" "-하고" is one of the conjunctive particles and "-도" is one of the auxiliary particles. 수요일'에' 빵하고 우유를 사요 is more natural this sentence is perfect itself. However, "수요일에 빵도 우유도 사요" this sentence sounds weird a little bit Another information should be there before that sentence. if you say like this, it would be more natural and pefect. "월요일에 쥬스를 사요. 수요일에는 빵도, 우유도 사요" "I buy juice on Monday. I also buy bread and milk on Wednesday." (In this sentence, the verb '사다' is omitted after '빵도' ▶ 수요일에는 빵도 사고 우유도 사요)