You will be mailed about how you have to pay the mess advance beforehand.
Also, the mess advance is taken in the start. It is like a debit system where at the start of each semester some amount is recharged in your mess account and then amount is deducted on basis of what mess you eat in. At the end of semester, you have to pay for excess, if any.
Yes the student and parent can eat at the mess on 28th itself by paying a small amount that the mess charges per plate which is around Rs. 50/plate.