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.

I have never heard of such a thing. Then again with all due respect, its hygiene. Lol. Is she the care giver for baby? If it was me, id secretly shower baby later. Since baby us under her care, then no point arguing. Im sure both of you mean well for baby, its only a matter of different believes and perspective.