How will you explain to your mother in law that you're not a believer of the saying "babies should not take a bath on Tuesdays and Fridays"? I told her politely that babies needs to take a bath everyday,but she insist that its bad luck. She even got mad at me.

Did she explain why? Actually, I just smile and acknowledge all suggestions of the elders (whether my family or my husband's family). Because it will just create misunderstanding as they will insist that it worked during their time. You can do what you want because you are the mother. There's no need to follow what they say.