Does by March first mean BEFORE March first, or ON March first?

I'm very confused, for my senior trip the due date for the first installment of money is "BY march first" does this mean the money is due tonight? (The company we are going through has payment online, so we don't have to actually go to our travel leader to give the deposit.) or does it mean it's due tomorrow? We need 5 kids to go, and the 5th thinks the money isn't due today! I'm trying to convince her! help!